From FFXI Wiki

Items which have the Rare Rare designation are unique. A player may only have one of these such items in their possession at any time. This includes in your inventory, any "house" storage location, or on the auction house. This is often paired with the Exclusive Exclusive designation, which binds the item solely to your character.

When storing a Rare Rare item on a Storage Slip, or sending it via Delivery Box, another may be obtained.