Sometimes duplicate users appear in Confluence and Jira. Let’s see a real life example in Confluence.
It takes user accounts from 2 AD, with users migrated from one to another. One of the users had the username starting with a lowercase letter in the old AD, while in the new the username starts with a capital letter. The rest are the same. And now even though only one username exists, the old lowercase username still appears in searches. This is because someone clicked Administer user on this old username sometime in the past, and the database still keeps a userinfo entry for that. Search this database entry using old username as parameter and delete it. Now there should be no more duplicates.