I’ve just done the same thing. It’s very unclear in game, and I had no idea to look for the solution to a problem that didn’t exist. You don’t look for an answer until it’s too late.
Unique could mean many things, but I wouldn’t have jumped to this conclusion until I had ruled out quite a few others. Either I shouldn’t be able to buy a second item of the same type, or they should say I can’t equip it, and give me a proper reason. Because I can buy more than one, the item is already not “unique”.