Session zero is your friend.

Personally I am of the opinion that some party rivalry is good. You stole that on your own? It's yours, sure. It's not real stuff after all. Who cares?!

But if you are obviously just taking shit in full view of everyone, in a hostile location, with no intention to share...well now that's a dick move and won't be tolerated. Still a good RPing opportunity. I have actually told people before: "my character needs to learn this lesson so please teach it to them", before doing stuff like this. With the understanding in place, it's a cool RP opportunity.