It would be an epic episode of IASIP if Frank finds out he has a long lost twin brother. They find out because Mac and Charlie are just chilling in the bar, Arnold walks in and they literally think it's Frank. Mac and Charlie can't tell Frank and Arnold apart while Dennis and Dee claim it's the twin-gene that they can tell.

Throughout the episode it gets increasingly frustrating for Charlie and Mac because they'll be intending to talk to Frank but at Arnold instead and Frank/Arnold has to keep pointing out who is who.

Dennis: "This is retarded, how can you not clearly tell the difference between the 2 of them. They look nothing alike!"

Charlie: "YOU'RE SO FULL OF SHIT DUDE! IT'S LIKE I'M LOOKING AT FRANK NEXT TO A MIRROR"

Dee: "Yeeaah uh Charlie, Frank is like half the height of his brother. How are you not seeing this?!"

Mac: "Half the height? I mean, maybe Frank is slouching a little bit. Stand straight up Frank..."

[Frank stands more upright, which to the viewer is barely noticeable"

Mac and Charlie start yelling at the same time.

Charlie: "THEY'RE THE SAME GODDAMN HEIGHT THEY'RE THE SAME GODDAMN HEIGHT!"

Mac: "They're clones! There is no other way those 2 are clones!"