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!
The solution is to tap the Maps button, then Menu, and Delete duplicate maps.
This will search your map list for maps with identical names, and it then uses the edition number or date of publication to determine which one to delete.