Is it
(1) Truly slam-firing (i.e., the hammer has not dropped along w/ the bolt closing), or
(2) Is the hammer dropping upon closing?
If #1, It's a stuck/broken firing pin (should be easy to see looking at it protruding from the bolt face)
If #2, You need to get the trigger/sear interface reworked/hardened.
Neither is a major issue to fix.