I have my own rules -
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If I'm camping a monster and someone asks to share it, I let them.
If they don't ask and go ahead and attack, I offer to take turns.
If they don't accept this offer and are polite about it, I simply compete for the npc.
if they don't accept this offer and are rude about it, I respond in kind.
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If someone else is camping a monster and I want to fight it, I ask them to share.
If they decline politely, I usually walk off.
If they decline rudely, I compete for it.
If the harass me for this, I respond in kind.
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All of this applies both IC and OOC.
It really is rude to run up and kill something (especially repeatedly) that someone else is already intending to kill, it doesn't hurt to ask them to share :/ And if they're not the type to share, then they're probably going to be rude about it, giving you the moral high ground if you choose to attack regardless.
We do not see 2 people standing next to a resource pressing macros to see who lucks out.
No, we don't. However let's say that the spawns become a place where a type of attackable npc often walks by. For example, the rogue spawn just out of east hydlaa just so happens to occasionally have a known criminal walk past it and there are multiple characters sitting waiting for this to happen. Whoever is quickest to begin an attack (keyboard shortcuts!) and can swing their weapon fastest (weapon speed!) is going to get the kill. This could be completely random.
It's possible for one of these characters to want the killing to themselves, in which case they might challenge the others like you said. However they might not, and just continue as they are, knowing that they are the quickest, or understanding that this place does not "belong" to them and they do not need to chase off anyone else who tries to use it for the same purpose.
I don't consider this to be unrealistic.