Are event accessory items unique, or can we use 2 of the same accessory?
As far as I have seen, all accessories can be worn twice (I.E. 2 ribbons, 2 hairpins, etc.).
Accessories and equipment can be exclusive to certain characters, but it will tell you that under it's secondary effects (when looking at the item's stats).
The only items that are unique would be Materia. Those usually have a small number 1 on the corner, showing you can only equip 1 of them, and it will tell you in the item's description.