If even a single bit is different in two images, Google Photos won't see them as duplicates. If you are seeing what look like duplicates in your Google Photos account, they are most likely either near duplicates or they are copies of the same photo that were changed in some way (such as, opening it in something like Photoshop and then re-saving it).

Photo backup client will hide/merge duplicate photos and will only show one copy of a duplicate. So, it will look like it is uploading the uploaded photos again, but it will just cycle through (re-scan) all the photos and only upload those that were not previously uploaded. If they have, it will skip that photo upload and check the next one and so on. The de-duplicating feature will check/scan to see if they have been previously uploaded (each photo file has a unique 'hash' that allows us to determine this). Google Photos has identical duplicate detection, which means that if you have uploaded photos to Google Photos already, it will not re-upload the same photo. Google Photos Backup Has Identical Duplicate Detection I think I will find a better software and keep things local.' This product will have cost me more time than I choose to think about. Nice try, Google, but you'll need to put better engineers on this one. Duplicates all over the place and no way to organize via tags.

'This service is full of bugs for uploading existing photos. 'Removing each one individually would take DAYS, and this issue has now ruined my online albums for sharing with friends and family.' You think it’s ok to create hours of work because you didn't take the time to explain the process before we created thousands of duplicates? You suck, and now I'll go use some other photo site.' People really are going to go elsewhere.' 'I can check the files myself and see that they are identical, but if you put a file from one computer on another computer it gets uploaded as a duplicate. Complaints of Google Photos Backup Duplicates

Mac photos check for duplicates