Phineas: Well, at least I'm evenly matched.
Leela: We're exactly the same, I know all her moves. Therefore, I have the upper hand. Hiiyyyya-ow!
(Leela and her Alternate Universe counterpart do the exact same kick and knock each other unconscious)
Professor Farnsworth: Now, now; perfectly symmetrical violence never solved anything.
McGee: All right. Ooh, okay - Gibbs versus...
(Gibbs walks up behind Abby)
Abby: Oh, oh that's good! Okay, so um, evil twin or a clone?
Jecht: I get to fight with the best fighter in the world!
"Aw, man, SHADOW Homestar? 'Hey, I've got a wonderful idea! Let's dip the main character in a vat of ink and make you fight him before you get to the end boss!'"
— Homestar, Homestar Runner, "Hremail 2000"
Snake: Colonel, it's me! I'm fighting myself!
Colonel: Snake, what's going on out there?! Could it be...? Has the "Les Enfants Terribles" project really come this far?!
Snake: It's more than that! He's got my moves, my gear... It's like looking in a mirror!
Colonel: Do you think they collected data on you, and created another Snake?
Snake: He's got the same build, too. Creepy. But his Sneaking Suit is a different color. And his tactics are slightly different.
Colonel: Interesting. So even if you're evenly matched in power, your fighting styles will make a big difference. You've faced tougher odds in the past, Snake. Don't let this impostor beat you!
You know, I remember reading somewhere that there are only two truly meaningful things you can do with your doppelganger when you're locked in a room. You can fight them... Or f#@k them.
— Brian O'Donovan, to Dan O'Brien, Agents Of Cracked' note
If I can defeat myself I will become complete!
Twilight: They're changelings, remember?
Fake Twilights: They're changelings, remember? (evil smile)
Yellow Lantern: Sanctimony is easy when you know what-
(one fight later)
Green Lantern: Well, that was surreal.
Wonder Woman: And a bit disturbing.
Insurgency Batman: You're good.
Main Universe Batman: You expected otherwise?
Insurgency Batman: Call it a draw?
Main Universe Batman: Not our style.
Regime Superman: I saved this world!
Main Universe Superman: Now I'll save it from you.
Regime Solomon Grundy: Like looking in mirror!
Main Universe Solomon Grundy: Grundy hates mirrors!
Main Universe Solomon Grundy Grundy think you look handsome!
Regime Solomon Grundy So does Grundy!
Main Universe Joker (Draws crowbar) Got a joke for you.
Regime Joker (Draws crowbar) I've probably heard it.
— Character clash quotes from Injustice: Gods Among Us