Glen has a ton of images on his iPhone and some are duplicates. How can he get rid of them? Apple says he has to delete them one at a time. If Glen has them backed up to iCloud with the iCloud Photo Library, he can enable "optimize disc space" on his phone. That will replace the full-size versions with smaller versions on the iPhone, while iCloud keeps the full-size versions. But once he deletes them, they get deleted from iCloud as well. This is why Leo recommends Google Photos, which offers to automatically delete images off the phone once they're backed up to Google Photos.
The problem is, it's really hard to tell if a duplicate really is a duplicate or not. There are apps he can run that will analyze them and remove them, but it can be risky. It's not perfect. The only sure way is to do it manually.