Hi @JackThor
While it is possible to have multiple login_ids associated with a user, there is only one first_name, last_name, full_name, sortable_name, or short_name for a user.
It is possible that at some point the user's name go manually edited, which would prevent an SIS feed file from updating it. However, you can get around that by checking the "Override UI changes" checkbox when manually uploading the feed file with the user's information.
The other alternative is that, if both of the login_ids associated with the user have SIS_ids (meaning, they can be updated via an SIS user feed), then there could be two records for the user included in your SIS user feed. If that is the case, then the name associated with the last record for the user in the SIS feed is the one that is left when the feed is done.