For Unique NPCs or Enemies, see Unique Encounters
If an item is flagged with UNIQUE in its description, a character can only have one of the said items; a second such item cannot be traded or picked up or bought. That is, you can have only one such item, no matter if it is in your inventory, backpack or bank. They cannot be sold through player shops. Backpacks with Unique items in them cannot be traded, even if the potential receiver only has one of them.
There are a few items with the same name and different IDs (like the Notum Rings of the Three and the Platinum Rings of the Three). This applies only to the items' ID. If a new item's ID is different to one already owned, it can be picked up or accepted in trade, regardless of its name.
Conversely if different QLs of an item share the same ID, it is not possible to have more than one. However, some UNIQUEs gain different IDs at higher levels, and may also gain new names, for example the Wen-Wen master of the universe (A character may own up to seventeen Wen-Wens, one for each different name).