Usernames in issue attributes suddenly showing up with a #1 suffix after LDAP synchronization and causing various undesired effects

Steps to Reproduce

1. Set up an LDAP directory, and rank it higher than the internal directory
2. Create a user in the internal directory, called B
3. Rename user A in LDAP to B
4. Synchronise

Expected Results

1. B in the internal directory gets renamed to B#1 in the app_user table
2. A gets renamed to B in the app_user table
3. Everything associated with both users becomes merged as the highest priority user

Actual Results

1. User rename appears like it's occurred fine, in the app_user table the internal user becomes A#1, and the LDAP user becomes A
2. Everything associated with both users becomes merged as the highest priority user
3. JIRA inconsistently recognises the B#2 in different aspects of the interface, it doesn't show up in the User Browser, but shows in certain Issue Attributes e.g Assignee or Reporter.
4. It is confusing to most Admins to suddenly see issue attributes automatically changing to B#2 when they have created no such user.
5. You're not able to login as B#2 to automatically regain access to the issues.
6. You are not able to use B#2 in the basic search, but then it shows up in Advanced JQL.
7. If you do a basic search with assignee set to B, then switch to Advanced JQL, the interface will not switch back to Basic

Notes

Due to the way this handling works, you can't deliberately have a username that ends with # followed by any number. It gets created, and appears in the user list, but you can't find it in any user browser drop down, and if you try to assign anything to that user you get an error "User 'user#1' cannot be assigned issues".