Add exception when adding multiple entries to the same dictionary using

the primitive

kernel/modelverse_kernel/primitives.py

@@ -339,7 +339,9 @@ def list_len(a, **remainder):
     raise PrimitiveFinished(result)
 
 def dict_add(a, b, c, **remainder):
-    new_edge = yield [("CE", [a, c])]
+    is_here, new_edge = yield [("RD", [a, b]), ("CE", [a, c])]
+    if is_here is not None:
+        raise Exception("dict_add on existing value!")
     yield [("CE", [new_edge, b])]
     raise PrimitiveFinished(a)