diff --git a/compiler/src/dotty/tools/dotc/reporting/diagnostic/ErrorMessageID.java b/compiler/src/dotty/tools/dotc/reporting/diagnostic/ErrorMessageID.java
index 09ca64e7fa8e..d4920b5665a8 100644
--- a/compiler/src/dotty/tools/dotc/reporting/diagnostic/ErrorMessageID.java
+++ b/compiler/src/dotty/tools/dotc/reporting/diagnostic/ErrorMessageID.java
@@ -107,7 +107,9 @@ public enum ErrorMessageID {
     TailrecNotApplicableID,
     FailureToEliminateExistentialID,
     OnlyFunctionsCanBeFollowedByUnderscoreID,
-    MissingEmptyArgumentListID
+    MissingEmptyArgumentListID,
+    DuplicateNamedTypeParameterID,
+    UndefinedNamedTypeParameterID
     ;
 
     public int errorNumber() {
diff --git a/compiler/src/dotty/tools/dotc/reporting/diagnostic/messages.scala b/compiler/src/dotty/tools/dotc/reporting/diagnostic/messages.scala
index 74ace0a629f2..4305f9594b68 100644
--- a/compiler/src/dotty/tools/dotc/reporting/diagnostic/messages.scala
+++ b/compiler/src/dotty/tools/dotc/reporting/diagnostic/messages.scala
@@ -1865,4 +1865,18 @@ object messages {
           |Excluded from this rule are methods that are defined in Java or that override methods defined in Java."""
     }
   }
+
+  case class DuplicateNamedTypeParameter(name: Name)(implicit ctx: Context)
+    extends Message(DuplicateNamedTypeParameterID) {
+    val kind = "Syntax"
+    val msg = hl"Type parameter $name was defined multiple times."
+    val explanation = ""
+  }
+
+  case class UndefinedNamedTypeParameter(undefinedName: Name, definedNames: List[Name])(implicit ctx: Context)
+    extends Message(UndefinedNamedTypeParameterID) {
+    val kind = "Syntax"
+    val msg = hl"Type parameter $undefinedName is undefined. Expected one of ${definedNames.map(_.show).mkString(", ")}."
+    val explanation = ""
+  }
 }
diff --git a/compiler/src/dotty/tools/dotc/typer/TypeAssigner.scala b/compiler/src/dotty/tools/dotc/typer/TypeAssigner.scala
index 45c8a422b855..338f0c2da0bf 100644
--- a/compiler/src/dotty/tools/dotc/typer/TypeAssigner.scala
+++ b/compiler/src/dotty/tools/dotc/typer/TypeAssigner.scala
@@ -371,9 +371,9 @@ trait TypeAssigner {
           val namedArgMap = new mutable.HashMap[Name, Type]
           for (NamedArg(name, arg) <- args)
             if (namedArgMap.contains(name))
-              ctx.error("duplicate name", arg.pos)
+              ctx.error(DuplicateNamedTypeParameter(name), arg.pos)
             else if (!paramNames.contains(name))
-              ctx.error(s"undefined parameter name, required: ${paramNames.mkString(" or ")}", arg.pos)
+              ctx.error(UndefinedNamedTypeParameter(name, paramNames), arg.pos)
             else
               namedArgMap(name) = preCheckKind(arg, paramBoundsByName(name.asTypeName)).tpe
 
diff --git a/compiler/test/dotty/tools/dotc/reporting/ErrorMessagesTests.scala b/compiler/test/dotty/tools/dotc/reporting/ErrorMessagesTests.scala
index ca3bc6b06585..1f8c65a874f7 100644
--- a/compiler/test/dotty/tools/dotc/reporting/ErrorMessagesTests.scala
+++ b/compiler/test/dotty/tools/dotc/reporting/ErrorMessagesTests.scala
@@ -1088,4 +1088,48 @@ class ErrorMessagesTests extends ErrorMessagesTest {
       val MissingEmptyArgumentList(method) :: Nil = messages
       assertEquals("method greet", method.show)
     }
+
+  @Test def duplicateNamedTypeParameter =
+    checkMessagesAfter("frontend") {
+      """
+        |object Test {
+        |  def f[A, B]() = ???
+        |  f[A=Any, A=Any]()
+        |  f[B=Any, B=Any]()
+        |}
+        |
+      """.stripMargin
+    }
+    .expect { (ictx, messages) =>
+      implicit val ctx: Context = ictx
+
+      assertMessageCount(2, messages)
+      val DuplicateNamedTypeParameter(n2) :: DuplicateNamedTypeParameter(n1) :: Nil = messages
+      assertEquals("A", n1.show)
+      assertEquals("B", n2.show)
+    }
+
+  @Test def undefinedNamedTypeParameter =
+    checkMessagesAfter("frontend") {
+      """
+        |object Test {
+        |  def f[A, B]() = ???
+        |  f[A=Any, C=Any]()
+        |  f[C=Any, B=Any]()
+        |}
+        |
+      """.stripMargin
+    }
+      .expect { (ictx, messages) =>
+        implicit val ctx: Context = ictx
+
+        assertMessageCount(2, messages)
+        val UndefinedNamedTypeParameter(n2, l2) :: UndefinedNamedTypeParameter(n1, l1) :: Nil = messages
+        val tpParams = List("A", "B")
+        assertEquals("C", n1.show)
+        assertEquals(tpParams, l1.map(_.show))
+        assertEquals("C", n2.show)
+        assertEquals(tpParams, l2.map(_.show))
+
+      }
 }