When something good happens twice in a row.
on getting a star twice in a row on Mario Kart 64:
"Two stars! Double plumsey!"
Two of something rare in a row (either good or bad).
A: "Did you see that guy who got stuck by lightning two days in a row?"
B: "Poor guy, a lightning double plumsey is bad luck."
A: "I was playing poker last night and I got a full house."
A: "Yeah, and then I got another full house straight after!"
B: "No way?! Double plumsey."