ok, email wasn’t the best example. It was more like that. Bob registered 1 month ago. Then he forgot he is registered and start a new registration. (Happens many times)
but based on your answer that’s what I need: if the mutation was executed I get the uid back. if not uids are empty.
(I think the response from your example is if you do an RDF request. But I do a JSON request)
I checked what I get in api.Response and there Response.Json is empty. there is nothing in.
But at least Response.Uids works 
Thanks again