They say there are only 7 basic jokes and that all humour is based around those in some fashion. So it's inevitable that 2 humorists will eventually hit upon the same joke at the same time completely independently. Isn't it?
From my list of regularly read comics (down there on the left), both Sinfest and Least I Could Do hit upon the same punchline today......
Oh, and did I mention that I've added Least I Could Do to my list of regular comics (and removed Acid Keg because it hasn't been updated for about 5 months).