In the event The Shadow Lord Invades, there is a Shantotto-specific accessory called the Golden Hairpin. Since our tiny sorc has 2 accessory slots like everyone else, does this mean I can craft 2 hairpins given enough materials, and if equipping them onto the same character will their effects stack?
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.
I did some searching on event information, and people are discussing equipping Shantotto with 2 hairpins, robes and a rod (all her exclusive items) which indicates that the hairpin is not unique for a slot.
Note this answer only addresses the specific example I had in mind, but I don't know if other event accessories may be limited to 1 per character (or does FFBE even have such a modifier on equipment?)