I quick flipped through the BRB and found no limiting rule in the Alliance or the magic item area. The Unique rule on pg 500, does say "you can only have onr of each in YOUR army unless otherwise stated in your army unless otherwise stated in thr magic item's rules.". It goes on to say that common means common to all armies, which make me think you can have duplicate if the magic item is in army A and army B, but army B can't have two of the same item.
Hope this helps