There's a Mythos card (Museum Haunted, with the Curse Of The Dark Pharaoh expansion) which places 2 ghosts in Miskatonic University and states "Investigators who destrony one of these Ghosts before the end of the next turn may draw one Exhibit Item".
So if one Investigator destroys one Ghost then evades the other, he draws one Exhibit Item. Then if another Investigator comes along and destroys the other Ghost, he'd also get an Exhibit Item.
But what if one Investigator destroys both? How many Exhibit Items does he get?