When you download a package of maps or charts more than once, and they are from a different year of publication than the original ones you downloaded, then you may have a lot of duplicate charts listed in the app. It is hard to tell which is the latest one to use, and would be a lot of work to delete them one by one!
On the PC, the best way to solve this is to exit Memory-Map Navigator, and use the Windows File Explorer to go to C:\Maps\DMS\Charts and delete the folder of charts that you no longer want.