If you go to the imports folder, it will show you all the images you imported on that day. You could just delete the entire import (But ONLY if the images are still on your phone) and then try it again, just importing new ones.
Or if they are not on your phone, you could add all of that import to an album. In the album you can then sort by date, and will be able to then delete all from that import which are already in the library (Presumably older ones up to a particular date). (To delete from an album - rather than just remove from the album, you need to hold down the CMD key while doing it)
As alsways when deleting a lot of images you should make sure you have an up to date backup first.
There are paid apps that will detect and help you to delete duplicates, but if this is just one import they are probably un-necessary.