The possibility of there being a duplicated customer user exists at the time of this article being published. Checks for this condition were made and a few accounts were seen to have some duplicates. This likely occurred in the past and it was not caught by some older legacy code. In most cases, one or more of the records were marked as "deleted", but still physically exist in order to preserve actual user info when displayed in logs, etc.
In these cases, one of the user accounts is the "real" account and is the one we want to keep. Thus, once GTMA IV is rolled out, we will go through these duplicates and remove them based on some criteria like age of the account, when last used.
In the event that a duplicate is removed and the user cannot login, the best thing to do will be to lookup their account on the customer user screen and simply send a new invite email or a password reset email. They can then reset their login password and access their account like normal.
Once these duplicates have all been resolved, we will remove this help article as there will not be a way for duplicates to exist in the new release.