You know that gag in the episode "The Slumber Party", with the girl that looked exactly like Spongebob named Girly Teengirl? Did anyone else think that was a rip-off of that gag from The Simpsons with the guy that looked exactly like Homer named Guy in Cognito?

Well you have to keep in mind they've done a very similar thing in SBSP before, in "No Weenies Allowed" they did almost the same thing, so it would have to depend on whether the aforementioned Simpsons episode was before or after the production of No Weenies Allowed.

And not everyone would notice that as The Simpsons has so many episodes

As RedSoxFan274 said, it reminded me more of the drifter on No Weenies Allowed, basically, it was the exact same gag. And they've done that on tons of shows, like on Family Guy when Peter took a free cookie sample at the supermarket, then a japanese version of Peter passed by, and then another one with a hat, you thought it was all Peter with disguises but then you see that it's just guys identical to Peter. Sane thing with SB, the drifter and Girly.

I was so sure that the girl who looked like SpongeBob was just SpongeBob in disguise, but it was so funny when I found out she actually wasn't SpongeBob.

Well, as you can see, the bit from NWA is in my siggy.