The action to create a user in the database does not (seem to) work if the proposed entry is a duplicate. I cannot find a way of catching that exception and using it to inform the user and maybe ask another question, for example, “user already registered; would you like to add user to your contacts?”
I have tried using the “This action will only happen if…” feature in the “Create” action, and following it with a Modal action. But with or without the “This action will …”, the execution “hangs” if the user to be added is a duplicate.