BUG-38723 add trait to an entity which already has the trait throws a 500

repository/src/main/java/org/apache/hadoop/metadata/services/DefaultMetadataService.java

@@ -331,6 +331,9 @@ public class DefaultMetadataService implements MetadataService {
         // ensure trait type is already registered with the TS
         Preconditions.checkArgument(typeSystem.isRegistered(traitName),
                 "trait=%s should be defined in type system before it can be added", traitName);
+        // ensure trait is not already defined
+        Preconditions.checkArgument(getTraitNames(guid).contains(traitName),
+                "trait=%s is already defined for entity=%s", traitName, guid);
 
         repository.addTrait(guid, traitInstance);