Does Photos have to be open and minimized or closed for it to find duplicates?

Hello malcflo,

Thanks for posting in Apple Support Communities. If you are using macOS Ventura, your duplicate photos will automatically appear in the Duplicates album. This should happen in the background and you don't have to do anything in particular within the app. As mentioned in Remove duplicate photos in Photos on Mac, you can then merge any duplicates found by doing the following.

1. In the Photos app  on your Mac, click Duplicates in the sidebar.
2. Select the duplicates you want to merge.
3. Click Merge [number] Duplicates. 

One original source photo or video appears where the duplicates were located.

Cheers!

The library needs to be open. When not using it directly must move it to the background (or minimized).