Did you create new event for the next year event?
That was my first question when I saw your post, Clayton. My guess is that last year's event was just updated with the new date instead of creating a new event. That would explain why (assuming that
"prevent duplicate registration" is also enabled) old registration data is preventing users from registering again.
The best fix for this situation would be to open your event in the Administrator backend and "Save as Copy", rename the new and old event to appropriate names and close registration for the old event, manually re-assign any registrations for this year that have been successfully completed on the old event to the new copy, and then either unpublish the old event or reset it's dates to last year's dates so that it goes back into an past-event status. This will also create a new URL for the event and you'll have to update any links that are out there and/or set up a link or auto-redirect on the old event page's URL to forward people to the new URL. You can program an auto- redirect via your site's .htaccess file or by installing a Joomla component such as ReDJ.
If creating a copy is not an option you would 1) have to delete all past registration records from the event, or 2) turn off prevent duplicate registration (but that will set your registration count waaaay off as last year's records will also show up, and it could result in duplicate registration within any of your upcoming events).
