How do I remove numerous duplicate photos on my iMac?

The safest method is to compare your photos and if you have redundant photos keep the photo with the best quality (pixel size, sharpness, codec). Only you can decide which is the best version of several very similar images.

There are quite a few duplicate detectors around, but some are not safe to use with a Photos Library. They may cause library damage and data loss. Apple is warning against such tools: Using third-party apps to remove duplicate photos might damage your Photos for macOS library - Apple Support

Avoid duplicate detectors or cleaners, that remove items directly from the library and do not just collect them in an album for you to review.

Power Photos and Photo Sweeper have been working well for me.