Deleting a mailbox and moving aliases to another mailbox assigns the mailbox to the same alias twice

[Sherlock1979]

Prerequisites

• [x] I have searched open and closed issues to make sure that the bug has not yet been reported.

Bug report

Describe the bug Using the web interface...

1. Define two mailboxes. E.g. [email protected] and [email protected]
2. Create an alias (e.g. [email protected]) and assign both mailboxes to it.
3. Delete one of the mailboxes (e.g. m2) and tell it to move the aliases to the other mailbox (m1).
4. On the Domains page you can see the correct number of total aliases (e.g. 1). But on the Mailboxes page you will see 2 aliases for m1.
5. On the Aliases page you cannot see any discrepancies but using the Android app you can see that the same mailbox (e.g. m1) is assigned twice to the same alias.

Expected behavior When you delete a mailbox and move the aliases to another mailbox, SL should not assign the mailbox to an alias that is already assigned to that mailbox