When I am killing things, I don't usually look at them once I am famililar with them. I just wander up to them and kill them. So it is very easy to accidently kill a monster that someone has already started to kill but had to flee from.
Sometimes it is very obvious coz the battle is too easy. Someone will usually say "Hey! did you just kill so-and-so" and when asked "politely", the spoils of battle can be shared.
However, what does one do if you have a full battle, collect the spoils and then someone says "Hey, I was killing so-and-so - I want whatever they dropped". Sure it can lead to interesting role-play but in the end, what does one do - hand over the loot or tell the other person to find a cliff and jump off it