Alexa: You're twins.
A Twin Trope. The tendency of twins to banter with each other. This includes Finishing Each Other's Sentences and feeding each other lines, among other things. A common trait of Single-Minded Twins and Telepathic Twins.
This can also be done by other multiple siblings, or even Those Two Guys.
Truth in Television, especially with twins of the same gender, as sharing one room and being in the same grade means they spend much more time together than average siblings. For that matter, some sets of twins develop a language specific to themselves that no one else understands.
- The Duumvirate does this without sentence-finishing.
- Fred and George Weasley from Harry Potter, naturally.
- Who could forget Tweedledum and Tweedledee?
- Jeremy and Ian from Barbie and the Diamond Castle. They even banter in song.
- Kaoru and Hikaru in Ouran High School Host Club do this regularly.
- Jim and Tim Possible.
- Tomax and Xamot, of G.I. Joe.
- The Corsican Brothers
- Castor and Pollux Stone, The Rolling Stones by Robert A. Heinlein. Also Laz and Lor in his later novels.
- Shion and Mion from Higurashi no Naku Koro ni do this occasionally, but sometimes it isn't always for laughs.
- Gem and Gemma from Power Rangers RPM are an extreme case, as it is exceedingly rare for them to individually speak in complete sentences- usually they alternate words or phrases.
- The Belgariad's Beltira and Belkira. "We're sorry, it's - " "Our nature."
- Bakemonogatari's Next Time On sequences are rapid-fire, seemingly random exchanges between Tsukihi and Karin, the main character's younger sisters.
- Real Life example: Tegan & Sara.
- The Sklar brothers of Cheap Seats (Also a staple of their stand-up routine)
- The Real Life comedy duo, Stone and Stone. Sibling Yin-Yang and Incest Subtext ahoy.
- Ethan and Evan in Dalton
- Marian and Stephanie from Fragile, while far from single-minded, occasionally are "doing some weird twin-chat".
- Eerie Cuties has sisters of Cessily (and as such snow fairies) Neige and Sorbet.