Google-photos – How To Edit Title Caption

If you have the Google Photos saved in your Google Drive, you should be able to install the Google Drive for Mac / PC to get the folder 'mounted' on your computer.

Google Drive Installer

From there, you can use an application such as dupeGuru (which works on Mac and Windows). You should be able to scan the Google Photos folder within your Google Drive and find the duplicates.

dupeGuru Actions -- Do whatever you want with your duplicates. Not only can you delete duplicates files dupeGuru finds, but you can also move or copy them elsewhere. There are also multiple ways to filter and sort your results to easily weed out false duplicates (for low threshold scans).

dupeGuru

While on Google Photos click Upload, (probably upper right corner), then click "Google Drive". If for some reason that didn't work, just read this short official Google help made to answer this exact question, it will tell you what's wrong.

If after clicking upload "google drive" doesn't show up, It's probably because your account is work/school. One workaround is to share the photos to a non-educational account, and follow the method given at the beginning of this answer.