What is the right way to let Photos find duplicates?

Photos doesn't want to slow down your work with tons of background funcitons-- and there's lots. It's scanning all the pictures in the Library not just to find duplicates, but to find squirrels and landmarks and faces and all kinds of stuff. It does some of that while you're paused thinking of what to do next, for instance, but mostly it waits for when you're not trying to use the app.

Photos also doesn't want to drain your battery, so it tries to do most of the background work while the Mac is plugged in.

So, the usual suggestion is to plug the Mac in with good internet connection, and with Photos in the background. Photos can be on, but it should not be being used, so you might have Mail up front, for instance. But you can also quit Photos -- it's never really quiet. If we know there is background work to be done, we usually leave the Mac on overnight with the display told to not sleep-- you set this in Settings>Battery>Options.

How's that?

Quitting Photos is fine. But don't log out or put the Mac to sleep. (There are some questions about this, by the way, but there's really nothing to lose.) In Battery>Options

Since ⌘⌃Q gives the login screen, which could allow other users to log on, I'm not sure that would continue a particular user's background processes.

Actually, some suggestions do show up. More suggestions on something probably make it more likely-- unless it's something they really don't want to do.