I have encountered this bug 2 times myself. When I had 2 First Aid Kits in my backpack (at both times it was a hunters backpack, not expecting that has anything to do with it though) I'm fine while staying in the same server I found the second one (or both). But when I log out and rejoin a server (both times I logged out for quite a while and didn't really pay attention if I relogged on the same server, but I expect not) one of the First Aid Kits had disappeared from my inventory.