Disabling Copy Imported Photos
As mentioned, Photos copies a photo by default when you add it to your Photos Library. This means that when you drag a photo from the Finder into Photos, the original photo remains in the Finder. The photo that shows up in Photos is a separate copy of that original file. In other words, you have two copies of that same photo.
Photos gives you the option of turning off the Copy Imported Photos feature in the Photos preferences. By turning it off, any photo you import into Photos will automatically be removed from its original location after import.
Disable Copy Imported Photos
Select Photos, Preferences from the menu bar at the top of your screen.
On the General tab, uncheck the Copy Items to the Photos Library box.