If I have two copies of a photo and want to delete one, how do I know which album that photo will be deleted from?
That is a very good question. Photos cannot reveal the album a photo is in.
I am using the work-around to add a keyword with the name of the album to photos, when I add them to an album. This way, I can see in the Info panel in the keyword section the name of the albums, where I added that photo to.
You could use an Apple Script to show a list of albums, for a selected photos - see this user tip: How to find out the albums a photo is in: Jacques Rioux's script