The other day down in the mines, another person invited me into his party, then went to clear the delivery quest with the Foreman. When he did, I got the closure journal entry for the quest, thus (I assume as I didn't actually try) stopping me from doing it myself, later. I didn't get any exp, though.
If it's of any use, they were already two doing it at the time (which might be a related bug, as most delivery quests are solo ones, right?).