What are the differences between merge options in Photos app?

Filenames aren't unique, so using filenames, especially generic ones, often yields multiple hits. Instead of IMG_3609.jpg, it seems sometimes to work better searching for3600, for some reason. In this case, I'm not sure what problem you are trying to solve.

If you want more control, you should try the trusted 3rd party app PowerPhotos ($30) There, you can set up very specific rules in searching for duplicates, like this:

set to save the older of two duplicates. You can add many steps to a rule.

PowerPhotos does much more than duplicates, like managing multiple Libraries, providing sortable lists, etc. Many of us couldn't get far without it.

I use Lightroom to edit RAW files, but I use Photos for curating and organizing pictures.

Some scanning is totally local in Photos. Facial scanning used to be entirely local, I think, but some information now seems to sync. The thing is, there's no place in metadata formats for face locations. It's the same with object recognition, whichI think is local. And scanning for duplicates seems to produce different results locally on different devices. I have not special knowledge about this-- just some experience

You have 2 images that are exact duplicates. The others are perceived to be duplicate images but may vary in size, resolution, etc.

Merge the 2 and you can review the rest if you wish

When you Merge two pictures, one of them goes to Recently Deleted. So it's not lost-- you can undo it.

So why not select 3 rows of duplicates (for instance) and see what happens? Then let us know…