I did some looking into this and it seems that WP has a history of returning duplicate users when doing a user query.
https://www.google.com/search?client=firefox-b-1-d&q=wp+user+query+returns+multiples+of+same+user
Sorry, I don’t have a fix for you. Most of what I’ve found says it was fixed but you’ve obviously found a case that it is not. After following some of the stuff the search turned up what I would probably to is build a list in an array of users that have already been shown and then skip the duplicates and work around the problem.
