[SPARK-19940][ML][MINOR] FPGrowthModel.transform should skip duplicated items

## What changes were proposed in this pull request?

This commit moved `distinct` in its intended place to avoid duplicated predictions and adds unit test covering the issue.

## How was this patch tested?

Unit tests.

Author: zero323 <zero323@users.noreply.github.com>

Closes #17283 from zero323/SPARK-19940.

Author: zero323

mllib/src/main/scala/org/apache/spark/ml/fpm/FPGrowth.scala

@@ -245,10 +245,10 @@ class FPGrowthModel private[ml] (
             rule._2.filter(item => !itemset.contains(item))
           } else {
             Seq.empty
-          })
+          }).distinct
       } else {
         Seq.empty
-      }.distinct }, dt)
+      }}, dt)
     dataset.withColumn($(predictionCol), predictUDF(col($(featuresCol))))
   }
 

mllib/src/test/scala/org/apache/spark/ml/fpm/FPGrowthSuite.scala

@@ -103,6 +103,20 @@ class FPGrowthSuite extends SparkFunSuite with MLlibTestSparkContext with Defaul
       FPGrowthSuite.allParamSettings, checkModelData)
   }
 
+  test("FPGrowth prediction should not contain duplicates") {
+    // This should generate rule 1 -> 3, 2 -> 3
+    val dataset = spark.createDataFrame(Seq(
+      Array("1", "3"),
+      Array("2", "3")
+    ).map(Tuple1(_))).toDF("features")
+    val model = new FPGrowth().fit(dataset)
+
+    val prediction = model.transform(
+      spark.createDataFrame(Seq(Tuple1(Array("1", "2")))).toDF("features")
+    ).first().getAs[Seq[String]]("prediction")
+
+    assert(prediction === Seq("3"))
+  }
 }
 
 object FPGrowthSuite {