Had been working with Trend Micro's Dr Cleaner, which has duplicate photo tool, but with recent security concerns we have removed it.

Other tools on MacOS app store seem somewhat shady, with small no-name software publishers, so I am wary to give such wide access right to personal data.

Will probably try coding up something in Python, but looking for other suggestions?

Also, do native macos Photos de-duplicate on import (so if you have same photo in two folders, and when you import to photos it identifies the 2nd copy and doesn't import into photo library)? I have read that may be the case (and only works on import) but really unclear from documentation.