Duplicate entries for users in user picker fields

Problem Definition

While attempting users to the Watchers field in any issues, users appear as duplicates (and the occurrence increases) in the suggested user list.

Environment

1. The affected users could be users from External Directories, for example, Microsoft Active Directory with mix case or uppercase in their username
2. It also can be done with User Management's Edit User action on Internal Directory as long as username is mixed case.

Steps to Reproduce the Problem

1. Log in to JIRA as the AD user with mixed case (problem is including Active Directory but not limited to it)
2. Open another browser which does not share the same session as step 1, log in as an admin.
3. Go to any existing issue and click to Add Watchers.
4. Type the name of the AD user in the Watcher field but do not add it.
5. Log out as the AD user and log in again.
6. As an admin, refresh the issue page and attempt to add the AD user as a watcher again.

Expected Result

There will be no duplicated users in the suggested user list.

Actual Result

The username appear as duplicates on the user picker. With every logout and login, the usernames adds an additional duplicate entry on the user picker.

Workaround

Restart JIRA.

As soon as users start performing login cycles again, the number of occurrences in user pickers will increase again.