"Import All New Photos" in Photos.app imports duplicates
Repairing my Photos.app Library fixed the issue! Photos/videos that I've imported now show up as "Already Imported" instead of as "New Photos".

The Solution


  Hold ⌥-⌘ (Option-Command) while launching Photos.app


You will be prompted to repair your library. Proceed to do so.

Note: It is highly recommended to back up your library first in case the repair process corrupts it.

Source (Apple Forums Q&amp;A)