Problem with multiple domains

I have a server (17.07) with multiple domains. (Ubuntu)
domain1, domain2, domain3.
Authentication is set to SQL. MySQL.

If I register a user like this:
ejabberdctl register newuser domain1 password

It creates 3 new users, one on each domain.
I end up with newuser@domain1, newuser@domain2, newuser@domain3.

If I remove any one of those, then all of them get removed.
Can anyone advise what to check to fix this?