Problem assigning a user to an issue when same userkey in 2 directories

When the same userkey appears in 2 different user directories, and those 2 users have different group access, it can sometimes not be possible to assign that user to an issue (as can not be searched for in user picker).

Steps to reproduce

1. Create 2 users in different user directories with same username (should also then be same userkey)
2. The user within the directory id with lowest id (normally JIRA internal) is not a member of the service-desk-agents group
3. The other user is a member of the group
4. The group with highest id (probably not the JIRA Internal) is higher in order of priority
5. Try to assign that user to a Service Desk issue that they should be an agent in

Expected

• The user can be assigned to the issue

Actual

• The user can not be found or assigned