There is no way to automatically remove duplicates from Google Photos
To avoid duplicates, Google Photos does not allow you to download an exact copy of an already saved file. On the other hand, if you have modified a photo, the modified version will be synchronized since it no longer looks like the original. If the photos in your Google Drive are also set to sync with Photos, you'll see duplicates of whatever was already in your library. Fortunately, it is possible to manually remove synced duplicates and turn off syncing photos on Google Drive.
Steps
Method 1 of 3: Disable syncing photos on Google Drive

Step 1. Open Google Photos on your computer, phone, or tablet
If the photos saved to your Google Drive are set to sync with Google Photos, you'll see duplicate photos that were already in your library. To prevent photos in Google Drive from appearing in Google Photos, you can turn off this feature.
- On your computer, sign in to this page to access Google Photos.
- If you are using a phone or tablet, tap the multicolored flower icon with the wording “Google Photos” (on iPhone and iPad) or “Photos” (on Android), respectively on the home screen and in the app drawer.

Step 2. Click or press ≡
This button is located at the top left and opens a menu.

Step 3. Select Settings

Step 4. Disable the Google Drive option
Scroll down to the switch Google drive then slide it to the Off position

. The switch is located under the header Google. If your Drive contained duplicate files that were already in Google Photos, those versions will be deleted.
The photos will not be deleted from your Google Drive
Method 2 of 3: Remove duplicates on a computer

Step 1. Log in to this page
If you are not yet logged in, enter your email address and password.
Google Photos will not sync identical photos, even if they are from different devices. However, if you have modified a synchronized photo (for example by cropping it, adding the location or modifying the date), the modified version will be synchronized since it is no longer identical to the original. You can avoid this by saving the edited photos to a folder that you won't sync with Google Photos

Step 2. Go to the Photos tab
This tab is on the left of the page and opens a list of all your photos.

Step 3. Find the photo you want to delete
Scroll through the photos until you find the one you want to delete.

Step 4. Click the photo check mark
Hover your mouse cursor over the photo thumbnail and click the check mark in the upper left corner.
Be careful not to select the 2 photos in a pair of duplicates

Step 5. Click the trash can icon

This icon is located at the top right of the page. A confirmation message will appear.

Step 6. Select Move to Trash
This option is in the confirmation message and allows you to put the photos in the recycle bin. Photos will remain in the recycle bin for 60 days in case you change your mind.
Method 3 of 3: Remove duplicates on the mobile app

Step 1. Open the Google Photos app
The app icon looks like a green, yellow, red, and blue flower and is usually found on the home screen or in the app drawer.
Google Photos won't sync duplicates, even if they come from different devices, but if you've edited a synced photo (whether you've cropped it, added a location, or changed the date), the version modified will be synchronized because it is no longer identical to the original. To prevent this, you can save the edited photos to a folder that you won't sync with Google Photos

Step 2. Go to Photos
This option is at the bottom left.

Step 3. Select ⁝ Where ⋯.
This button is located at the top right of the screen and opens a menu.

Step 4. Press Select Where Select photos.
This option is at the top of the menu.

Step 5. Choose the photos you want to delete
As you tap photos, the circles visible at the top left will change to check marks.
Make sure you don't delete the 2 photos in a duplicate pair

Step 6. Tap the delete icon

This icon is located at the top right of the screen and opens a confirmation window.

Step 7. Choose DELETE
The photos will be placed in the recycle bin where they will remain for 60 days in case you realize that you have made a mistake.