You probably have a few dozen duplicate files, chiefly MP3 and photos in the Google Drive. This is a handy tool that scans selected directories and finds any duplicate files inside them. The app works with all sorts of files and can also be configured to scan sub-folders within a given directory. You can make judgement yourself if you want to trash by selecting them. It also provides Shared File (owned by me) Finder, Orphaned File Finder (Not have a parent folder). Supports IE, Chrome, Firefox... Provides connect with Google Drive. You can directly scan duplicate files with your drive.
Google Photos tries to avoid exact duplicates, but does not always succeed when different upload methods or devices are used. Also photos which have been edited or metadata changed may be uploaded again. A different title/path does not trigger a new upload.
Also important: the Google Photos app on mobile devices not only shows photos/videos uploaded to Google Photos, but also photos in folders on the device. That may under certain circumstances (for example moving photos to another folder, adding photos to the device) lead to duplicates in the Photos tab. To see what is really uploaded to Google Photos you should use the web app //photos.google.com/
When we get new info on how to avoid/remove duplicates we will add it here.
New! //www.androidcentral.com/google-photos-more-like-this-button
Reasons for getting duplicates
If you uploaded using Picasa, a new upload to Google Photos created duplicates of all photos that were modified in Picasa, for example edited, date & time changed, tags added, etc. Renaming a photo did not result in duplicates.
Uploading from multiple devices, for example, a smartphone and computer. EXACT copies of photos will not be duplicated, but VIDEOS will be duplicated.
When switching to a new phone, some people reported seeing duplicates. It's not clear why or how this triggers duplicates.
New or reformatted computer may cause duplicates.
It may upload RAW & JPEG or HEIC & JPEG
Many asked for a de-duplication program, but Google says that "it cannot decide which photo the user wants to keep". What would already help is a search option to display duplicates side-by-side so that the user can more easily decide.
In order to find out where the duplicates come from you can check a few things:
Check in the info field in the web app //photos.google.com/ where each copy comes from, for example "Uploaded from a browser", "Uploaded from an Android (or iOS) device", "Shared by Name, saved to your Photos", etc
Check in the "recently added" field (under "Explore" in the web app and under "Search" in the mobile app) whether both are uploaded at the same day or not.
Do both duplicates have the same or different size ? Smaller copies may be thumbnails.
Your camera may be set to take both RAW and JPEG
Sometimes the same photo is shown twice. You may find out because deleting one also deletes the other. You can confirm by adding a "Description" to one of the duplicates: it's not a real duplicate when the Description is alsoadded to the other photo.
Recommendations to remove duplicates
There is no easy way to remove duplicates in Google Photos (prevention is important!), but this may help a bit:
You can delete duplicates manually, for example in //photos.google.com/ where they should be next to each other except when date/time was changed.
Duplicates may also appear together in albums created by Google under People, Places and Things, but you have to be careful: sometimes a photo is shown twice. When you add for example something in the Description field", and it also appears in the second photo, there is in fact only one photo, and deleting one will delete both.
You may have to check which ones you can delete:
One photo may be edited, the other not. You need to open the photo and click the edit icon to find out.
Deleting a photo removes it from ALL albums. One photo may be in an album, the other not or in more/different albums. You can see in the info panel to which album(s) a photo is added and add the photo you want to keep to ALL albums.
One photo may have more pixels or more KB or MB (see info of the photo)
It may be easier to delete batches of duplicates in //photos.google.com/search/_tra_ where they are sorted by upload date (most recent on top). That separates two sets of same photos so that you can select a range to delete.
You can also add a bunch of photos and duplicates to an album (it can have 20.000 photos now), and sort by upload date (Open the album > click "More options" (3-dots ) > click "Sort" (2 arrows) > "Recently added". That will group the photos by upload date, making it possible to select multiple photos and delete them in batch. It is possible to delete photos directly from an album (in web app only). Go to "More options" (3-dots icon), and use "Move to bin".
When you uploaded in the free "High Quality" (or converted to "High Quality" using "Recover storage" in //photos.google.com/settings) and have all your photos in albums, you could ignore the duplicates in the library, since they do not use your storage. This will change on June 1, 2021. The files uploaded with setting HQ before that date will still not count for storage, but new files will.
Duplicate removal in Google Photos:
//duplicates-google-photos.remikel.fr/
tutorial //youtu.be/ULalSu3MPRE
See also //support.google.com/photos/thread/91700588?hl=en&msgid=92347853
Duplicate removal after download to a computer
Some users suggested programs to delete duplicates on a computer hard disk. See here how to download photos from Google Photos to a computer.
Picasa3 (still used by many) has the option to display duplicates close to each other, thus facilitating manual removal. Tools > Experimental > Show Duplicate Files
//blogs.systweak.com/11-duplicate-photo-finder-tools-to-delete-duplicate-photos/
//www.visipics.info
similarityapp.com
//www.bolidesoft.com/imagecomparer.html
//www.duplicatephotocleaner.com/
//www.ashisoft.com/
//www.joerg-rosenthal.com/en/antitwin/
Duplicate removal on a mobile device
Google Files: //support.google.com/files/answer/9764075
Android: //www.google.com/search?q=remove+duplicates+android
iOS: //www.google.com/search?q=remove+duplicates+iOS