Using Cleaver it's possible to kill monsters twice.

[NHTangles]

A couple of scenarios can cause this:

1. Long worms (the bug factory that keeps on giving) - Using cleaver, hit multiple segments of the long worm and it can die more than once, leaving multiple corpses and worm teeth.
2. Gas spores (noticed on the Beholder level) Hit multiple gas spores and they can be killed multiple times (by both cleaver and each-other's explosions). Other monsters in the swing arc can also die twice (from cleaver and gas spore), and leave duplicate corpses.

Both scenarios result in an impossible() and potentially duplicate corpses, etc, but no serious impact.

Apologies if this is already a known issue.

Cheers
Tangles.