Unique

Unique items are those items which any player can only wield or wear one of
at a time.  Those items which are listed as unique in status will typically
be extremely high in level, such as artifact items.  Occasionally a
thematically appropriate item will be listed as a unique item as well.

Unique items are not restricted in terms of how many can be in the game. 
They are only restricted in terms of individual player use.