how do i remove photo duplicates

There is no automatic tool in Photos to remove duplicates from the library. Photos is scanning for duplicates when we import photos, so there should not be duplicate image files and no tool for the removal necessary.

Try to detect duplicates by looking at the All Photos view, where the images are sorted by the date and duplicates should be appearing side-by-side. Third-party apps for the detection and removal of duplicates are not all safe to use and might damage your Photos Library seriously and cause data loss. Apple is warning against them: Using third-party apps to remove duplicate photos might damage your Photos for macOS library - Apple Support

Power Photos is safe to use, because it is not modifying the library directly, and only using the interface of Photos to check for duplicates, but you need the paid, full version.

For dealing with duplicates in Photos (and many other things) check out PowerPhotos

https://www.fatcatsoftware.com/powerphotos/