Correct. Only one allowed across all inventory space per character. You could not have one in stash and one in inventory.
It was also not possible to have 2 unique charms in a trade window. If you wanted to trade a torch for a different torch, you had to drop trade them.
I remember doing drop trades on the lut gholein docks. One person stood at the top and one on the bottom, idea was you would both drop the item and have to run around to take the other one. It was really fun to do that and have other player just teleport down with their enigma and take your item while you were running up, happened a few times to me… 