Duplicate user in the user picklist (Cache issue)

Summary

Scenario

Considering you are going to add a second LDAP directory to your Jira instance, by creating or re-enabling it, and you have a user with same 'username' in both LDAPs. Once you create/enable and sync the second LDAP, you may notice users appearing twice in the in the user picklist when:

1. Adding a user to a role
2. Trying to add the user as a watcher, reporter, assignee, etc.

Steps to Reproduce

1. Install JIRA 7.6.4 or newer
2. Create sample business project
3. Connect to an external directory containing users
4. Attempt to assign an issue to one of the users in the external directory
   • Observe: Assignee search (picklist) works and shows the users
5. Connect and sync the second external directory containing users with the same 'username'
6. Attempt to assign an issue to one of the users with the same 'username' in both the external directory
   • Observe: Assignee search (picklist) will show duplicate users. Other fields such as 'Watcher' and 'User Role'.

Expected Results

Jira should only show only the user entry in the topmost directory.

Actual Results

Users are duplicated in the cached and as a result, user appear duplicate in user picker after the directory is created/enabled

Workaround

Recommended workaround:

• Restart JIRA with both external directories enabled

Alternate workaround:

• Create new user directory then delete that new directory.
  • The act of deleting a directory will cause related caches to be reset.
• The directory does not need to function.
  • Create a new directory, fill out required fields. Click "Save and Test" once to get a connection error, click again to save the config.
  • After that disable the new directory then delete it.