Apple software to remove duplicate photo's on my Mac

Apple does not provide a tool to remove duplicates.

You have either to find duplicates yourself or use (safe) third-party tools to find duplicates and collect them for you in an album, where you can delete them safely using the Photos.app.

Ironically, Photos 5 and Photos 6 are spending a lot of time to identify duplicates and similar photos, but Photos is using this information to hide the duplicates from the curated "Days" view, so the duplicates will not bother us any longer. We can use this as a starting point to collect all photos in the Days in an album, then create a smart album showing the Photos not in the DAYS album to find candidates for Duplicates. See: About the Duplicate Detection in Photos 5 - Apple Community

I found these two third-party Duplicate Detectors to be safe: Power Photos and Photo Sweeper. But both are not free, we need the full, paid versions to help us to remove duplicates.