diff --git a/docs/sql-migration-guide.md b/docs/sql-migration-guide.md
index 5a5e802f6a90..ffa55765abc0 100644
--- a/docs/sql-migration-guide.md
+++ b/docs/sql-migration-guide.md
@@ -49,7 +49,7 @@ license: |
 
   - In Spark version 2.4 and earlier, float/double -0.0 is semantically equal to 0.0, but -0.0 and 0.0 are considered as different values when used in aggregate grouping keys, window partition keys and join keys. Since Spark 3.0, this bug is fixed. For example, `Seq(-0.0, 0.0).toDF("d").groupBy("d").count()` returns `[(0.0, 2)]` in Spark 3.0, and `[(0.0, 1), (-0.0, 1)]` in Spark 2.4 and earlier.
 
-  - In Spark version 2.4 and earlier, users can create a map with duplicated keys via built-in functions like `CreateMap`, `StringToMap`, etc. The behavior of map with duplicated keys is undefined, e.g. map look up respects the duplicated key appears first, `Dataset.collect` only keeps the duplicated key appears last, `MapKeys` returns duplicated keys, etc. Since Spark 3.0, these built-in functions will remove duplicated map keys with last wins policy. Users may still read map values with duplicated keys from data sources which do not enforce it (e.g. Parquet), the behavior will be undefined.
+  - In Spark version 2.4 and earlier, users can create a map with duplicated keys via built-in functions like `CreateMap`, `StringToMap`, etc. The behavior of map with duplicated keys is undefined, e.g. map look up respects the duplicated key appears first, `Dataset.collect` only keeps the duplicated key appears last, `MapKeys` returns duplicated keys, etc. Since Spark 3.0, new config `spark.sql.legacy.allowDuplicatedMapKeys` was added, with the default value `false`, Spark will throw RuntimeException while duplicated keys are found. If set to `true`, these built-in functions will remove duplicated map keys with last wins policy. Users may still read map values with duplicated keys from data sources which do not enforce it (e.g. Parquet), the behavior will be undefined.
 
   - In Spark version 2.4 and earlier, partition column value is converted as null if it can't be casted to corresponding user provided schema. Since 3.0, partition column value is validated with user provided schema. An exception is thrown if the validation fails. You can disable such validation by setting `spark.sql.sources.validatePartitionColumns` to `false`.
 
diff --git a/python/pyspark/sql/functions.py b/python/pyspark/sql/functions.py
index e80d556cc89e..b7ff7264aa18 100644
--- a/python/pyspark/sql/functions.py
+++ b/python/pyspark/sql/functions.py
@@ -2766,12 +2766,12 @@ def map_concat(*cols):
     :param cols: list of column names (string) or list of :class:`Column` expressions
 
     >>> from pyspark.sql.functions import map_concat
-    >>> df = spark.sql("SELECT map(1, 'a', 2, 'b') as map1, map(3, 'c', 1, 'd') as map2")
+    >>> df = spark.sql("SELECT map(1, 'a', 2, 'b') as map1, map(3, 'c') as map2")
     >>> df.select(map_concat("map1", "map2").alias("map3")).show(truncate=False)
     +------------------------+
     |map3                    |
     +------------------------+
-    |[1 -> d, 2 -> b, 3 -> c]|
+    |[1 -> a, 2 -> b, 3 -> c]|
     +------------------------+
     """
     sc = SparkContext._active_spark_context
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/collectionOperations.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/collectionOperations.scala
index 6ed68e47ce7a..2b123479a190 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/collectionOperations.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/collectionOperations.scala
@@ -516,8 +516,8 @@ case class MapEntries(child: Expression) extends UnaryExpression with ExpectsInp
   usage = "_FUNC_(map, ...) - Returns the union of all the given maps",
   examples = """
     Examples:
-      > SELECT _FUNC_(map(1, 'a', 2, 'b'), map(2, 'c', 3, 'd'));
-       {1:"a",2:"c",3:"d"}
+      > SELECT _FUNC_(map(1, 'a', 2, 'b'), map(3, 'c'));
+       {1:"a",2:"b",3:"c"}
   """, since = "2.4.0")
 case class MapConcat(children: Seq[Expression]) extends ComplexTypeMergingExpression {
 
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/ArrayBasedMapBuilder.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/ArrayBasedMapBuilder.scala
index 98934368205e..3848d573ddf6 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/ArrayBasedMapBuilder.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/ArrayBasedMapBuilder.scala
@@ -20,6 +20,8 @@ package org.apache.spark.sql.catalyst.util
 import scala.collection.mutable
 
 import org.apache.spark.sql.catalyst.InternalRow
+import org.apache.spark.sql.internal.SQLConf
+import org.apache.spark.sql.internal.SQLConf.LEGACY_ALLOW_DUPLICATED_MAP_KEY
 import org.apache.spark.sql.types._
 import org.apache.spark.unsafe.array.ByteArrayMethods
 
@@ -48,6 +50,9 @@ class ArrayBasedMapBuilder(keyType: DataType, valueType: DataType) extends Seria
   private lazy val keyGetter = InternalRow.getAccessor(keyType)
   private lazy val valueGetter = InternalRow.getAccessor(valueType)
 
+  private val allowDuplicatedMapKey =
+    SQLConf.get.getConf(LEGACY_ALLOW_DUPLICATED_MAP_KEY)
+
   def put(key: Any, value: Any): Unit = {
     if (key == null) {
       throw new RuntimeException("Cannot use null as map key.")
@@ -63,6 +68,11 @@ class ArrayBasedMapBuilder(keyType: DataType, valueType: DataType) extends Seria
       keys.append(key)
       values.append(value)
     } else {
+      if (!allowDuplicatedMapKey) {
+        throw new RuntimeException(s"Duplicate map key $key was founded, please check the input " +
+          "data. If you want to remove the duplicated keys with last-win policy, you can set " +
+          s"${LEGACY_ALLOW_DUPLICATED_MAP_KEY.key} to true.")
+      }
       // Overwrite the previous value, as the policy is last wins.
       values(index) = value
     }
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala
index b0be37d2b2ee..5a63a9e9d623 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala
@@ -2167,6 +2167,16 @@ object SQLConf {
     .booleanConf
     .createWithDefault(false)
 
+  val LEGACY_ALLOW_DUPLICATED_MAP_KEY =
+    buildConf("spark.sql.legacy.allowDuplicatedMapKeys")
+      .doc("When true, use last wins policy to remove duplicated map keys in built-in functions, " +
+        "this config takes effect in below build-in functions: CreateMap, MapFromArrays, " +
+        "MapFromEntries, StringToMap, MapConcat and TransformKeys. Otherwise, if this is false, " +
+        "which is the default, Spark will throw an exception when duplicated map keys are " +
+        "detected.")
+      .booleanConf
+      .createWithDefault(false)
+
   /**
    * Holds information about keys that have been deprecated.
    *
diff --git a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/CollectionExpressionsSuite.scala b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/CollectionExpressionsSuite.scala
index 9e98e146c7a0..01df6675016d 100644
--- a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/CollectionExpressionsSuite.scala
+++ b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/CollectionExpressionsSuite.scala
@@ -139,8 +139,10 @@ class CollectionExpressionsSuite extends SparkFunSuite with ExpressionEvalHelper
       MapType(IntegerType, IntegerType, valueContainsNull = true))
     val mNull = Literal.create(null, MapType(StringType, StringType))
 
-    // overlapping maps should remove duplicated map keys w.r.t. last win policy.
-    checkEvaluation(MapConcat(Seq(m0, m1)), create_map("a" -> "4", "b" -> "2", "c" -> "3"))
+    withSQLConf(SQLConf.LEGACY_ALLOW_DUPLICATED_MAP_KEY.key -> "true") {
+      // overlapping maps should remove duplicated map keys w.r.t. last win policy.
+      checkEvaluation(MapConcat(Seq(m0, m1)), create_map("a" -> "4", "b" -> "2", "c" -> "3"))
+    }
 
     // maps with no overlap
     checkEvaluation(MapConcat(Seq(m0, m2)),
@@ -272,8 +274,10 @@ class CollectionExpressionsSuite extends SparkFunSuite with ExpressionEvalHelper
     checkEvaluation(MapFromEntries(ai1), create_map(1 -> null, 2 -> 20, 3 -> null))
     checkEvaluation(MapFromEntries(ai2), Map.empty)
     checkEvaluation(MapFromEntries(ai3), null)
-    // Duplicated map keys will be removed w.r.t. the last wins policy.
-    checkEvaluation(MapFromEntries(ai4), create_map(1 -> 20))
+    withSQLConf(SQLConf.LEGACY_ALLOW_DUPLICATED_MAP_KEY.key -> "true") {
+      // Duplicated map keys will be removed w.r.t. the last wins policy.
+      checkEvaluation(MapFromEntries(ai4), create_map(1 -> 20))
+    }
     // Map key can't be null
     checkExceptionInExpression[RuntimeException](
       MapFromEntries(ai5),
@@ -294,8 +298,10 @@ class CollectionExpressionsSuite extends SparkFunSuite with ExpressionEvalHelper
     checkEvaluation(MapFromEntries(as1), create_map("a" -> null, "b" -> "bb", "c" -> null))
     checkEvaluation(MapFromEntries(as2), Map.empty)
     checkEvaluation(MapFromEntries(as3), null)
-    // Duplicated map keys will be removed w.r.t. the last wins policy.
-    checkEvaluation(MapFromEntries(as4), create_map("a" -> "bb"))
+    withSQLConf(SQLConf.LEGACY_ALLOW_DUPLICATED_MAP_KEY.key -> "true") {
+      // Duplicated map keys will be removed w.r.t. the last wins policy.
+      checkEvaluation(MapFromEntries(as4), create_map("a" -> "bb"))
+    }
     // Map key can't be null
     checkExceptionInExpression[RuntimeException](
       MapFromEntries(as5),
diff --git a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/ComplexTypeSuite.scala b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/ComplexTypeSuite.scala
index 9039cd645159..2c1e0c846046 100644
--- a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/ComplexTypeSuite.scala
+++ b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/ComplexTypeSuite.scala
@@ -21,6 +21,7 @@ import org.apache.spark.SparkFunSuite
 import org.apache.spark.sql.catalyst.analysis.{TypeCheckResult, UnresolvedExtractValue}
 import org.apache.spark.sql.catalyst.dsl.expressions._
 import org.apache.spark.sql.catalyst.expressions.codegen.CodegenContext
+import org.apache.spark.sql.internal.SQLConf
 import org.apache.spark.sql.types._
 import org.apache.spark.unsafe.types.UTF8String
 
@@ -216,10 +217,12 @@ class ComplexTypeSuite extends SparkFunSuite with ExpressionEvalHelper {
       CreateMap(interlace(strWithNull, intSeq.map(Literal(_)))),
       "Cannot use null as map key")
 
-    // Duplicated map keys will be removed w.r.t. the last wins policy.
-    checkEvaluation(
-      CreateMap(Seq(Literal(1), Literal(2), Literal(1), Literal(3))),
-      create_map(1 -> 3))
+    withSQLConf(SQLConf.LEGACY_ALLOW_DUPLICATED_MAP_KEY.key -> "true") {
+      // Duplicated map keys will be removed w.r.t. the last wins policy.
+      checkEvaluation(
+        CreateMap(Seq(Literal(1), Literal(2), Literal(1), Literal(3))),
+        create_map(1 -> 3))
+    }
 
     // ArrayType map key and value
     val map = CreateMap(Seq(
@@ -281,12 +284,14 @@ class ComplexTypeSuite extends SparkFunSuite with ExpressionEvalHelper {
       MapFromArrays(intWithNullArray, strArray),
       "Cannot use null as map key")
 
-    // Duplicated map keys will be removed w.r.t. the last wins policy.
-    checkEvaluation(
-      MapFromArrays(
-        Literal.create(Seq(1, 1), ArrayType(IntegerType)),
-        Literal.create(Seq(2, 3), ArrayType(IntegerType))),
-      create_map(1 -> 3))
+    withSQLConf(SQLConf.LEGACY_ALLOW_DUPLICATED_MAP_KEY.key -> "true") {
+      // Duplicated map keys will be removed w.r.t. the last wins policy.
+      checkEvaluation(
+        MapFromArrays(
+          Literal.create(Seq(1, 1), ArrayType(IntegerType)),
+          Literal.create(Seq(2, 3), ArrayType(IntegerType))),
+        create_map(1 -> 3))
+    }
 
     // map key can't be map
     val arrayOfMap = Seq(create_map(1 -> "a", 2 -> "b"))
@@ -399,10 +404,12 @@ class ComplexTypeSuite extends SparkFunSuite with ExpressionEvalHelper {
     val m5 = Map("a" -> null)
     checkEvaluation(new StringToMap(s5), m5)
 
-    // Duplicated map keys will be removed w.r.t. the last wins policy.
-    checkEvaluation(
-      new StringToMap(Literal("a:1,b:2,a:3")),
-      create_map("a" -> "3", "b" -> "2"))
+    withSQLConf(SQLConf.LEGACY_ALLOW_DUPLICATED_MAP_KEY.key -> "true") {
+      // Duplicated map keys will be removed w.r.t. the last wins policy.
+      checkEvaluation(
+        new StringToMap(Literal("a:1,b:2,a:3")),
+        create_map("a" -> "3", "b" -> "2"))
+    }
 
     // arguments checking
     assert(new StringToMap(Literal("a:1,b:2,c:3")).checkInputDataTypes().isSuccess)
diff --git a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/HigherOrderFunctionsSuite.scala b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/HigherOrderFunctionsSuite.scala
index e7b713840b88..b3438538a336 100644
--- a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/HigherOrderFunctionsSuite.scala
+++ b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/HigherOrderFunctionsSuite.scala
@@ -465,8 +465,10 @@ class HigherOrderFunctionsSuite extends SparkFunSuite with ExpressionEvalHelper
     checkEvaluation(
       transformKeys(transformKeys(ai0, plusOne), plusValue),
       create_map(3 -> 1, 5 -> 2, 7 -> 3, 9 -> 4))
-    // Duplicated map keys will be removed w.r.t. the last wins policy.
-    checkEvaluation(transformKeys(ai0, modKey), create_map(1 -> 4, 2 -> 2, 0 -> 3))
+    withSQLConf(SQLConf.LEGACY_ALLOW_DUPLICATED_MAP_KEY.key -> "true") {
+      // Duplicated map keys will be removed w.r.t. the last wins policy.
+      checkEvaluation(transformKeys(ai0, modKey), create_map(1 -> 4, 2 -> 2, 0 -> 3))
+    }
     checkEvaluation(transformKeys(ai1, plusOne), Map.empty[Int, Int])
     checkEvaluation(transformKeys(ai1, plusOne), Map.empty[Int, Int])
     checkEvaluation(
diff --git a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/util/ArrayBasedMapBuilderSuite.scala b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/util/ArrayBasedMapBuilderSuite.scala
index 8509bce17712..87bbdb7300e4 100644
--- a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/util/ArrayBasedMapBuilderSuite.scala
+++ b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/util/ArrayBasedMapBuilderSuite.scala
@@ -20,10 +20,12 @@ package org.apache.spark.sql.catalyst.util
 import org.apache.spark.SparkFunSuite
 import org.apache.spark.sql.catalyst.InternalRow
 import org.apache.spark.sql.catalyst.expressions.{UnsafeArrayData, UnsafeRow}
+import org.apache.spark.sql.catalyst.plans.SQLHelper
+import org.apache.spark.sql.internal.SQLConf
 import org.apache.spark.sql.types.{ArrayType, BinaryType, IntegerType, StructType}
 import org.apache.spark.unsafe.Platform
 
-class ArrayBasedMapBuilderSuite extends SparkFunSuite {
+class ArrayBasedMapBuilderSuite extends SparkFunSuite with SQLHelper {
 
   test("basic") {
     val builder = new ArrayBasedMapBuilder(IntegerType, IntegerType)
@@ -42,64 +44,79 @@ class ArrayBasedMapBuilderSuite extends SparkFunSuite {
     assert(e.getMessage.contains("Cannot use null as map key"))
   }
 
-  test("remove duplicated keys with last wins policy") {
+  test("fail while duplicated keys detected") {
     val builder = new ArrayBasedMapBuilder(IntegerType, IntegerType)
     builder.put(1, 1)
-    builder.put(2, 2)
-    builder.put(1, 2)
-    val map = builder.build()
-    assert(map.numElements() == 2)
-    assert(ArrayBasedMapData.toScalaMap(map) == Map(1 -> 2, 2 -> 2))
+    val e = intercept[RuntimeException](builder.put(1, 2))
+    assert(e.getMessage.contains("Duplicate map key 1 was founded"))
+  }
+
+  test("remove duplicated keys with last wins policy") {
+    withSQLConf(SQLConf.LEGACY_ALLOW_DUPLICATED_MAP_KEY.key -> "true") {
+      val builder = new ArrayBasedMapBuilder(IntegerType, IntegerType)
+      builder.put(1, 1)
+      builder.put(2, 2)
+      builder.put(1, 2)
+      val map = builder.build()
+      assert(map.numElements() == 2)
+      assert(ArrayBasedMapData.toScalaMap(map) == Map(1 -> 2, 2 -> 2))
+    }
   }
 
   test("binary type key") {
-    val builder = new ArrayBasedMapBuilder(BinaryType, IntegerType)
-    builder.put(Array(1.toByte), 1)
-    builder.put(Array(2.toByte), 2)
-    builder.put(Array(1.toByte), 3)
-    val map = builder.build()
-    assert(map.numElements() == 2)
-    val entries = ArrayBasedMapData.toScalaMap(map).iterator.toSeq
-    assert(entries(0)._1.asInstanceOf[Array[Byte]].toSeq == Seq(1))
-    assert(entries(0)._2 == 3)
-    assert(entries(1)._1.asInstanceOf[Array[Byte]].toSeq == Seq(2))
-    assert(entries(1)._2 == 2)
+    withSQLConf(SQLConf.LEGACY_ALLOW_DUPLICATED_MAP_KEY.key -> "true") {
+      val builder = new ArrayBasedMapBuilder(BinaryType, IntegerType)
+      builder.put(Array(1.toByte), 1)
+      builder.put(Array(2.toByte), 2)
+      builder.put(Array(1.toByte), 3)
+      val map = builder.build()
+      assert(map.numElements() == 2)
+      val entries = ArrayBasedMapData.toScalaMap(map).iterator.toSeq
+      assert(entries(0)._1.asInstanceOf[Array[Byte]].toSeq == Seq(1))
+      assert(entries(0)._2 == 3)
+      assert(entries(1)._1.asInstanceOf[Array[Byte]].toSeq == Seq(2))
+      assert(entries(1)._2 == 2)
+    }
   }
 
   test("struct type key") {
-    val builder = new ArrayBasedMapBuilder(new StructType().add("i", "int"), IntegerType)
-    builder.put(InternalRow(1), 1)
-    builder.put(InternalRow(2), 2)
-    val unsafeRow = {
-      val row = new UnsafeRow(1)
-      val bytes = new Array[Byte](16)
-      row.pointTo(bytes, 16)
-      row.setInt(0, 1)
-      row
+    withSQLConf(SQLConf.LEGACY_ALLOW_DUPLICATED_MAP_KEY.key -> "true") {
+      val builder = new ArrayBasedMapBuilder(new StructType().add("i", "int"), IntegerType)
+      builder.put(InternalRow(1), 1)
+      builder.put(InternalRow(2), 2)
+      val unsafeRow = {
+        val row = new UnsafeRow(1)
+        val bytes = new Array[Byte](16)
+        row.pointTo(bytes, 16)
+        row.setInt(0, 1)
+        row
+      }
+      builder.put(unsafeRow, 3)
+      val map = builder.build()
+      assert(map.numElements() == 2)
+      assert(ArrayBasedMapData.toScalaMap(map) == Map(InternalRow(1) -> 3, InternalRow(2) -> 2))
     }
-    builder.put(unsafeRow, 3)
-    val map = builder.build()
-    assert(map.numElements() == 2)
-    assert(ArrayBasedMapData.toScalaMap(map) == Map(InternalRow(1) -> 3, InternalRow(2) -> 2))
   }
 
   test("array type key") {
-    val builder = new ArrayBasedMapBuilder(ArrayType(IntegerType), IntegerType)
-    builder.put(new GenericArrayData(Seq(1, 1)), 1)
-    builder.put(new GenericArrayData(Seq(2, 2)), 2)
-    val unsafeArray = {
-      val array = new UnsafeArrayData()
-      val bytes = new Array[Byte](24)
-      Platform.putLong(bytes, Platform.BYTE_ARRAY_OFFSET, 2)
-      array.pointTo(bytes, Platform.BYTE_ARRAY_OFFSET, 24)
-      array.setInt(0, 1)
-      array.setInt(1, 1)
-      array
+    withSQLConf(SQLConf.LEGACY_ALLOW_DUPLICATED_MAP_KEY.key -> "true") {
+      val builder = new ArrayBasedMapBuilder(ArrayType(IntegerType), IntegerType)
+      builder.put(new GenericArrayData(Seq(1, 1)), 1)
+      builder.put(new GenericArrayData(Seq(2, 2)), 2)
+      val unsafeArray = {
+        val array = new UnsafeArrayData()
+        val bytes = new Array[Byte](24)
+        Platform.putLong(bytes, Platform.BYTE_ARRAY_OFFSET, 2)
+        array.pointTo(bytes, Platform.BYTE_ARRAY_OFFSET, 24)
+        array.setInt(0, 1)
+        array.setInt(1, 1)
+        array
+      }
+      builder.put(unsafeArray, 3)
+      val map = builder.build()
+      assert(map.numElements() == 2)
+      assert(ArrayBasedMapData.toScalaMap(map) ==
+        Map(new GenericArrayData(Seq(1, 1)) -> 3, new GenericArrayData(Seq(2, 2)) -> 2))
     }
-    builder.put(unsafeArray, 3)
-    val map = builder.build()
-    assert(map.numElements() == 2)
-    assert(ArrayBasedMapData.toScalaMap(map) ==
-      Map(new GenericArrayData(Seq(1, 1)) -> 3, new GenericArrayData(Seq(2, 2)) -> 2))
   }
 }
diff --git a/sql/core/src/test/scala/org/apache/spark/sql/DataFrameFunctionsSuite.scala b/sql/core/src/test/scala/org/apache/spark/sql/DataFrameFunctionsSuite.scala
index 7fce03658fc1..05b8ca4ac729 100644
--- a/sql/core/src/test/scala/org/apache/spark/sql/DataFrameFunctionsSuite.scala
+++ b/sql/core/src/test/scala/org/apache/spark/sql/DataFrameFunctionsSuite.scala
@@ -651,8 +651,10 @@ class DataFrameFunctionsSuite extends QueryTest with SharedSparkSession {
       Row(null)
     )
 
-    checkAnswer(df1.selectExpr("map_concat(map1, map2)"), expected1a)
-    checkAnswer(df1.select(map_concat($"map1", $"map2")), expected1a)
+    withSQLConf(SQLConf.LEGACY_ALLOW_DUPLICATED_MAP_KEY.key -> "true") {
+      checkAnswer(df1.selectExpr("map_concat(map1, map2)"), expected1a)
+      checkAnswer(df1.select(map_concat($"map1", $"map2")), expected1a)
+    }
 
     val expected1b = Seq(
       Row(Map(1 -> 100, 2 -> 200)),
@@ -3068,11 +3070,13 @@ class DataFrameFunctionsSuite extends QueryTest with SharedSparkSession {
       checkAnswer(dfExample2.select(transform_keys(col("j"), (k, v) => k + v)),
         Seq(Row(Map(2.0 -> 1.0, 3.4 -> 1.4, 4.7 -> 1.7))))
 
-      checkAnswer(dfExample3.selectExpr("transform_keys(x, (k, v) ->  k % 2 = 0 OR v)"),
-        Seq(Row(Map(true -> true, true -> false))))
+      withSQLConf(SQLConf.LEGACY_ALLOW_DUPLICATED_MAP_KEY.key -> "true") {
+        checkAnswer(dfExample3.selectExpr("transform_keys(x, (k, v) ->  k % 2 = 0 OR v)"),
+          Seq(Row(Map(true -> true, true -> false))))
 
-      checkAnswer(dfExample3.select(transform_keys(col("x"), (k, v) => k % 2 === 0 || v)),
-        Seq(Row(Map(true -> true, true -> false))))
+        checkAnswer(dfExample3.select(transform_keys(col("x"), (k, v) => k % 2 === 0 || v)),
+          Seq(Row(Map(true -> true, true -> false))))
+      }
 
       checkAnswer(dfExample3.selectExpr("transform_keys(x, (k, v) -> if(v, 2 * k, 3 * k))"),
         Seq(Row(Map(50 -> true, 78 -> false))))