Why is my Lumia 830 keeping multiple copies of pictures in the library?

No I do not. They just appear after taking a single shot. Usually if you delete one, all disaapear.
 
@Nokia830BG it's probably because it's showing you the on phone photo and the one stored in onedrive.

In the photos settings there is a section Viewing and editing, and an option "Linked duplicates" Make sure it's on and the doubles should go away.