JIRA allow the same username to be created in the internal directory itself when the same username already exist in in the delegated LDAP

Summary

When you have a user already exist under Delegated LDAP in JIRA, JIRA still allow you to create the same username in JIRA under JIRA internal directory.

Environment

JIRA 7.6.1

Steps to Reproduce

1. Have 2 user directory (1 JIRA internal directory and the other Delegated LDAP, in this case directory order doesn't matter)
2. Have the user from Delegated LDAP into JIRA
3. So now the user show up in JIRA under Delegated LDAP in JIRA, we called it userA
4. Go to Create Users in JIRA, then create the same username, userA under JIRA internal directory

Actual Results

It allows you to create the same username without any issue, and the now userA in JIRA is showing under JIRA internal directory instead of Delegated LDAP.

Expected Improvement

It should throw an error message to prevent the same username to be created in JIRA internal directory

Notes

When you try to rename the latest username you had just created to a different username, it will transfer all history of the older user account to the newer user account as this will caused unnecessary hassle to the affected user.