So far it seems as if "usable" items (i.e. their name is yellow in your inventory, like coffer keys) will drop and be lost, whereas other items (like delkfutt key, fransisca, etc.) will just simply not drop.
Common examples of usable items (name is yellow) that drop and get lost:
Coffer keys
Mandy buds
Spells
Common examples of other items (name is white) that do not drop at all if everyone has it:
ENM60 JSE ammo
Fransisca
Testimonies
I've seen plenty of coffer keys, mandy buds, and rare spells drop and get lost, and there's screenshots of the keys above, so I don't think that really needs proving.
As for Fransisca, in my experience, it seems that, without fail, he *always* drops the axe if someone doesn't have it. He's never dropped when everyone had it, and always dropped if someone didn't have it. As for ENM60 JSE ammo, I know it isn't necessarily a common drop, but I've been doing ENM60 since it came out and have never again gotten the ammo since I got my first one. Could just be bad luck, but I think its because I already have it.
As for the testimonies, ever party in sky and get a RDM testimony every few fights until your entire party had them, then they just stopped dropping? Unless someone keeps dropping theirs, at least.
XI Wiki

