removing duplicates from photo library

It will depend on the kind of duplicates. Are your duplicates similar photos or exactly duplicate files?

• If you are having exact duplicates - that can be found by the identical capture date or filename, PowerPhotos can find them.
• If you have similar photos (same photo, but in different file formats or with adjustments applied), you need to use a program that does not only check the file attributes but compares the images based on histograms or bitmaps. Photos Sweeper can do that.

Stay away from programs that modify the Photos Library directly; they can damage the library and you may lose photos, see Apple's warning: Using third-party apps to remove duplicate photos might damage your Photos for macOS library - Apple Support

Assuming you mean a Photos library check out

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