From Homestar Runner Wiki
Duplicate characters is a phenomenon where a single character appears in more than one place at once in the same scene, and in general, any other characters present simply accept this as true. This listing excludes purely stylistic uses of multiple characters through artistic license, as seen in music videos and the like.
- A Decemberween Pageant — Strong Mad's hand can be seen moving both sets of waves back and forth, on opposite sides of the stage.
- Cheat Commandos — In the Cheat Commandos cartoon, some of the characters walk by twice.
- Decemberween Teen Girl Squad — Two Arrow'd Guys are seen rating the girls on their dives into the Gift Exchange Lion's mouth.
- Email pizzaz — Strong Bad conducts an interview of himself, in that he is asking the questions, and another "he" is answering them. Strong Bad seems to find this perfectly normal, even though the two Strong Bads interview, mock, yell at, plead with and bribe each other. However, the sight of two Strong Bads simultaneously makes The Cheat's head explode.
- Senorial Day — Bubs holds a Bubsotathon, in which Homestar throws confetti while on both sides of the screen at once. The Flash file shows that the one on the right wears a mustache.
- Main Page 23 — Rolling over Store will show Homestar facing away from you, saying the word "Store", as well as another Homestar walking in the background with a simple exchange of "Sup, man?" — "Sup?"
- Fall Float Parade — Two fez-wearing Homsars go past, of which Marzipan takes only casual notice.
- Email strong badathlon — An Easter egg shows two Homestar puppets in an advertisement for "Homestar vs. Homestar fights".
- Email unnatural — Two The Cheats are seen at the town meeting. One is to the right of Coach Z, and the other is to the left of Marzipan.
- Email mini-golf — After The Worm bites off half of Homestar's golf ball, another Worm pops out of it.
- Email web comics — In one comic, Stinkoman wishes he could challenge himself. A second Stinkoman (with 1-Up's voice) appears in order to fulfill his wish.
- Email shapeshifter — Three shapeshifted Strong Bads (one in the form of Bubs's legs, one in the form of a $100 bill, and one in the form of a balloon tiger) appear in one scene, which Strong Bad then describes as "confusing".
- Homestar Ruiner — After being humiliated, Homestar crashes at the House of the Brothers Strong, occupying the basement, the kitchen, and Strong Bad's Room all at the same time. In fact, Strong Bad has to kick all three Homestars out of the house.
- Main Page 25 — Homestar and The Exact Same appear.
- Twenty THANxty Six — Stinkoman appears to be buried in the ground with one leg sticking out and another Stinkoman appears next to it.
- Hremail 2000 — A third leg similar to Homestar's appears during the puppet show and when Homestar is playing basketball. Also, in an Easter egg Homestar fights "Shadow Homestar".
 Screenshots of Duplicate Characters