17 points 23 minutes ago
I can just answer right here.
Bullets travel at a certain velocity. Impacts are checked frame by frame as the projectile travels along its path. The issue here was that if the bullet happened to impact a target just at the very end of the segment for its frame, sometimes it would register as a hit on the same target the very start of the next frame as well.
The collision response function failed to properly ignore this double impact, causing the damage to be given twice.
This also explains why we've been having a fairly hard time reproducing the issue as well, since it only happened if you were shooting a target at certain very precise ranges (basically a multiple of a full frame of travel time at bullet velocity worth of distance from the shooter).