Could be something super basic like assigning each lock on-map with a custom UUID, then adding a lock's UUID to a 'picked_locks' list when picked for the first time. Reduce the XP return for any lock already on the list, return the full XP if a list is not on the list as of the first pick.
#Modifying/optimizing existing anti-lockpicking cheese
frosty nexus
stable urchin
I mean, XP is XP when picking a lock. But also, I agree that this would be effective in reducing 'anti-lockpicking cheese' (great phrase). and while a Set List would have minimal performance impact, there are a lot of locks, as well as locks being created, and there are a lot of players so servers may be affected.
It may be a better idea to globally just reduce the amount of XP that's received when lockpicking.