Fix for issues 2380 and 2381

compiler/java/com/google/dart/compiler/resolver/ResolveVisitor.java

Index: compiler/java/com/google/dart/compiler/resolver/ResolveVisitor.java
===================================================================
--- compiler/java/com/google/dart/compiler/resolver/ResolveVisitor.java	(revision 8249)
+++ compiler/java/com/google/dart/compiler/resolver/ResolveVisitor.java	(working copy)
@@ -12,12 +12,14 @@
 import com.google.dart.compiler.ast.DartNode;
 import com.google.dart.compiler.ast.DartParameter;
 import com.google.dart.compiler.ast.DartTypeNode;
+import com.google.dart.compiler.ast.DartTypeParameter;
 import com.google.dart.compiler.type.DynamicType;
 import com.google.dart.compiler.type.FunctionType;
 import com.google.dart.compiler.type.Type;
 import com.google.dart.compiler.type.Types;
 
 import java.util.ArrayList;
+import java.util.HashSet;
 import java.util.List;
 
 /**
@@ -50,10 +52,16 @@
   }
 
   final FunctionAliasElement resolveFunctionAlias(DartFunctionTypeAlias node) {
-    FunctionAliasElement funcAlias = node.getElement();
-    for (Type type : funcAlias.getTypeParameters()) {
-      TypeVariableElement typeVar = (TypeVariableElement) type.getElement();
-      getContext().getScope().declareElement(typeVar.getName(), typeVar);
+    HashSet<String> parameterNames = new HashSet<String>();
+    for (DartTypeParameter parameter : node.getTypeParameters()) {
+      TypeVariableElement typeVar = (TypeVariableElement) parameter.getElement();
+      String parameterName = typeVar.getName();
+      if (parameterNames.contains(parameterName)) {
+        getContext().onError(parameter, ResolverErrorCode.DUPLICATE_TYPE_VARIABLE, parameterName);
+      } else {
+        parameterNames.add(parameterName);
+      }
+      getContext().getScope().declareElement(parameterName, typeVar);
     }
     return null;
   }