Played in a circle the participants hit a beer can against their head twice with considerable impact (hence duff duff) and then pass the same beer can to their left. The next participant does the same and that continues until the can springs a leak.
The person whose head caused the leak then needs to open the can and down it.
The player to the loser's left takes a fresh can and the game continues.
Duff Duff is often played at men only events such as but not limited to: camping trips, bucks parties and festivals.