I’m betting the primary cause is a major flaw in the Motor Voter law. States are required to register you but there is no viable process to cancel any previous registrations. Registrants are not required to provide info about previous registrations and even if they do, notifications to cancel are haphazard at best. No wonder there are so many duplicates.
Many states would require you to re-register if you missed a general election and your name was removed. Now judges have prohibited states from purging voter rolls. If you want to find collusion look to federal judges who prohibited voter ID states from enforcing their laws.