--- a/graal/com.oracle.truffle.dsl.processor/src/com/oracle/truffle/dsl/processor/typesystem/TypeSystemParser.java	Fri Sep 06 16:11:15 2013 +0200
+++ b/graal/com.oracle.truffle.dsl.processor/src/com/oracle/truffle/dsl/processor/typesystem/TypeSystemParser.java	Fri Sep 06 16:16:40 2013 +0200
@@ -71,7 +71,7 @@
         }
 
         typeSystem.setTypes(parseTypes(typeSystem));
-        if (typeSystem.getTypes() == null) {
+        if (typeSystem.hasErrors()) {
             return typeSystem;
         }
 
@@ -91,6 +91,8 @@
         if (casts == null || checks == null || implicitCasts == null) {
             return typeSystem;
         }
+
+        typeSystem.setImplicitCasts(implicitCasts);
         typeSystem.setCasts(casts);
         typeSystem.setChecks(checks);
 
@@ -106,6 +108,7 @@
             cast.getTargetType().addTypeCast(cast);
         }
 
+        verifyImplicitCasts(typeSystem);
         verifyGenericTypeChecksAndCasts(typeSystem);
         verifyMethodSignatures(typeSystem);
         verifyNamesUnique(typeSystem);
@@ -113,6 +116,24 @@
         return typeSystem;
     }
 
+    private static void verifyImplicitCasts(TypeSystemData typeSystem) {
+        Set<TypeData> types = new HashSet<>();
+        Set<TypeData> duplicateSourceTypes = new HashSet<>();
+        for (ImplicitCastData cast : typeSystem.getImplicitCasts()) {
+            if (types.contains(cast.getSourceType())) {
+                duplicateSourceTypes.add(cast.getSourceType());
+            }
+            types.add(cast.getSourceType());
+        }
+        for (TypeData duplicateType : duplicateSourceTypes) {
+            for (ImplicitCastData cast : typeSystem.getImplicitCasts()) {
+                if (cast.getSourceType().equals(duplicateType)) {
+                    cast.addError("Duplicate cast source type %s.", Utils.getSimpleName(duplicateType.getPrimitiveType()), ImplicitCast.class.getSimpleName());
+                }
+            }
+        }
+    }
+
     private static void verifyGenericTypeChecksAndCasts(TypeSystemData typeSystem) {
         for (TypeData type : typeSystem.getTypes()) {
             if (!type.getTypeChecks().isEmpty()) {