Among other things, your iPad will search for duplicates when it is plugged in and locked. There is nothing that you need to do. If you elected to optimize photos (settings > photos), optimization will happen over time. You'll need some space on your device for syncing and optimizing to work properly. Depending on the size of your library, it may take several days to upload, optimize, and find duplicates.
Check how much space is used versus available (settings > general > iPad storage). I've seem some folks recommend having 5 percent to 10 percent of your total device space free for sync to work properly.
Check your iCloud sync progress (in photos tap library, scroll to the bottom, your item count and cloud status display).
Delete or offload lesser used items