Carlos Valdes on The Flash
What happens when you go to solve a murder...and the murderer turns out to be you? That's what happened to Cisco when his ex-girlfriend was murdered. It was actually him from another earth, and he ended up having to physically fight the guy before putting him away on Earth-19 for good, meaning Carlos Valdes had to fight Carlos Valdes. Bless those stunt teams!
Eliza Taylor on The 100
Eliza Taylor took on a new role as the much more carefree Josephine while Clarke, the character she's been playing for five seasons and leader of Earth's survivors, appeared to have been erased. After a long battle with herself inside her own head (it's a long story), Clarke prevailed, but Josephine was good fun while she lasted.
Elizabeth Gillies on Dynasty
After Nicolette Sheridan left the role of Alexis Carrington, Gillies is stepping in as the fresh-faced(?) version of the character, who recently survived a face full of fire and got some serious plastic surgery.
Sarah Paulson on American Horror Story
Paulson has played many, many characters over eight seasons of AHS, but Apocalypse saw her playing 3 at once: Ms. Venable, Billie Dean Howard, and the iconic supreme Cordelia.
Yael Grobglas on Jane the Virgin
For several seasons, Grobglas played both Petra, the rich and (sometimes) wealthy twin with the hot (ex) husband, and also Anezka, the less wealthy and slightly more evil twin with the pretty bad hair. Petra later killed Anezka (in self defense!), and the evil twin was no more.
Alexandra Chando on The Lying Game
Freeform once had a show about a foster kid who learned she had an identical twin sister with a living a life of luxury and convinced her to temporarily switch places. Both twins were played by Alexandra Chando.
Troian Bellisario on Pretty Little Liars
For the final season of the Freeform drama, sometimes, Spencer was actually her evil twin sister Alex. Eagle eyed fans could pretty easily tell when Spencer was not Spencer, but that's a testament to Bellisario's acting. We won't speak of her accent work, however.
Tatiana Maslany on Orphan Black
Maslany should teach classes on acting opposite yourself. She played, among others, Sarah, Cosima, Helena, Alison, and Rachel on a regular basis on the BBC America show Orphan Black and deserved multiple awards for pretty much every episode.
Nina Dobrev on The Vampire Diaries
Elena Gilbert was from a long line of doppelgangers, so it was only natural that Dobrev had a lot of parts to play. Elena and the much more evil Katherine got the most screentime, but Tatia and Amara were memorable too!
Sarah Michelle Gellar on Ringer
SMG played troubled identical twin sisters Bridget and Siobhan on this short-lived CW series.
Patton Oswalt on Agents of SHIELD
Oswalt played all of the Koenigs, the quadruplet SHIELD agents who were long thought to be robots, and turned out to just be brothers.
Lisa Kudrow on Friends
Phoebe Buffay's twin sister Ursula was just about the complete opposite of Phoebe, even if Phoebe did fall in love with a couple of her boyfriends/stalkers, and even if Ursula did pose as Phoebe when she became a pornstar.
Jeffrey Wright on Westworld
After spending episode after episode telling us all about park creator Arnold, the HBO drama revealed that Arnold was actually the man Bernard the robot was based on, and both are played by Jeffrey Wright, and no we can't explain it any further.
Jeffrey Tambor on Arrested Development
Tambor played identical twin brothers George and Oscar on the sitcom. While George Sr was in prison, Oscar moved in on his wife Lucille...
Anna Torv in Fringe
Anna Torv broke hearts everywhere as Olivia and Fauxlivia, the red headed alternate dimension version of her who stole her life for a time.
Maisie Richardson Sellers on Legends of Tomorrow
After Amaya chose to stay behind in her own village and time period on the time-traveling superhero drama, Sellers didn't go anywhere. A gender-fluid shapeshifter chose Amaya's form to manipulate the Legends and got stuck in that body, leaving us with the much cheekier Charlie in her place.