First of all, there is no silver bullet to duplicate photos. However, find following a few tips on how to remove duplicates. The Google Photos apps—the mobile app and Desktop Uploader—have automatic duplicate detection to prevent storing multiple copies of the same photo.https://productforums.google.com/forum/#!topic/photos
Save space on your hard disk by creating additional photo libraries and store some on external hard disk. You can then sync also that external hard disk with google photos to ensure you have always a backup in the cloud and never lose any of the precious memories.