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I've seen it posted a few times on this board and in SF76's guide that you will receive 350 Gold if you open a chest and already have the item. Well, it's true that you receive gold, but the amount varies. The total will be equal to 10 x your current level. This helps prevent some imbalance early on, like buying the Healing Pot for 10 G and making 340G profit, which is quite a lot at the start.
The game actually seems to use the same routine in all cases where it gives you an item, whether it's from talking to an NPC, opening a chest, or killing a boss, though I can't recall if it's ever possible to get the level*10 gold from anything other than opening chests without fiddling with memory.