Steel crowbars equipped in both weapon slots. One has a firecracker cap applied to it, the other one does not. If you attack with the naked crowbar, the firecracker cap on the other one will just disappear as if used (it is not actually used though, there's no effect). Easy enough to replicate.
This does not happen if different crowbars are equipped. If there's a tungsten crowbar in one slot and a steel crowbar in the other, everything works as it should.
I suspect this is caused by all crowbars of the same type technically counting as the same item. Crowbars are rather unique in this since they also have no durability, so two copies of the same type are truly indistinguishable. I haven't tested yet, but high chance this will also apply to CACS. It's also possible this may be the case for other enhancements put on other semi-unique weapons you can have more than one of, like shivs, boxing gloves, etc. but these have durability so hard to say. Will check later.