Treat duplicate predicate declarations as error.

Author: unp1

key.core/src/main/java/de/uka/ilkd/key/nparser/builder/FunctionPredicateBuilder.java

@@ -78,6 +78,9 @@ public Object visitPred_decl(KeYParser.Pred_declContext ctx) {
 
         if (lookup(p.name()) == null) {
             functions().add(p);
+        } else {
+            // weigl: agreement on KaKeY meeting: this should be an error.
+            semanticError(ctx, "Predicate '" + p.name() + "' is already defined!");
         }
         return null;
     }