duplicate photos

Is this still happening? If so, how are your settings?

In Photos>Settings>General,

Is "Use as System Photo Library" grayed out (indicating it is) or solid (indicating this isn't your system library)?

Is "Copy Item to the Photos Library" checked? (This is the default, indicating a "Managed Library")

In Photos>Settings>iCloud

Is "iCloud Photos" checked?

If it is, is "Optimize Mac Storage" check?

Is your Photos Library in the Pictures folder? or where?

Try these things:

• Restart the computer (of course, you've done this!)
• Rebuild your Photos Library--close Photos and re-launch by option-command-clicking the app icon.
• Make a new user and see if the same problem recurs with it
• Use another library if you have one, or make a small test Library with a few pictures in it and see if the same thing happens.
• Start in Safe Mode (different for different computers) This bypasses certain potentially disruptive processes.

           Start up your Mac in safe mode - Apple Support 

The idea is to find out if the problem is with the Library, with your account, or with caches and login items, and stuff like that. One of these steps may fix the problem, but their failure to work is also diagnostic.