In the MCU
For an extensive list including all the unnamed background characters, see here. I've chosen to focus just on the characters with memorable roles who I can find pictures of.
Laura Haddock plays Peter Quill's mother, Meredith Quill (Guardians of the Galaxy), as well as a female fan of Captain America in the 1940's (Captain America: The First Avenger).
Damion Poitier played as Thanos (The Avengers) and also plays one of Crossbones Henchmen (Captain America: Civil War).
The twin sisters Imogen and Isabella Poynton play Rhomann Dey's Daughter (Guardians of the Galaxy) as well as Lila Barton (Avengers: Age of Ultron).
Nathan Fillion plays Monstrous Inmate (Guardians of the Galaxy) as well as Simon Williams/Wonderman (Guardians of the Galaxy Vol. 2 deleted scene).
Douglas Robson plays the Horned Marauder (Thor: The Dark World) as well as the dark elf in The Collector's museum (Guardians of the Galaxy).
Seth Green plays Howard the Duck (Guardians of the Galaxy) as well as Expo Fan (Iron Man 2).
Joe Russo plays S.H.I.E.L.D. surgeon Dr. Fine (Captain America: The Winter Soldier) as well as psychiatrist Theo Broussard (Captain America: Civil War).
Benedict Cumberbatch plays Stephen Strange (Doctor Strange) as well as Dormammu (Doctor Strange).
Taika Waititi plays Korg (Thor Ragnarok) as well as the right head of Haju (Thor Ragnarok).
Stephen Blackehart plays the Knowhere Dispatcher that Drax incapacitates (Guardians of the Galaxy) as well as Brahl (Guardians of the Galaxy Vol. 2).
Clem So plays a Kyln Prisoner (Guardians of the Galaxy) as well as a Kamar-Taj Disciple (Doctor Strange). (Though I can imagine these being the same character.)
Kenneth Choi plays Howling Commando Jim Morita (Captain America: The First Avenger) as well as his grandson Principal Morita (Spider-Man: Homecoming).
Martin Starr plays a student at Culver University (identified in the novelization as Amadeus Cho) (The Incredible Hulk) as well as science teacher Mr. Harrington (Spider-Man: Homecoming). (Ignoring the novelization, I can can see these being the same character.)
Michelle Yeoh plays Aleta Ogord (Guardians of the Galaxy Vol. 2) and Ying Nan (Shang-Chi and the Legend of the Ten Rings).
Gemma Chan plays Minn-Erva (Captain Marvel) and Sersi (The Eternals).
Patton Oswalt plays the five Koenig brothers (Agents of S.H.I.E.L.D.) as well as Pip the Troll (Eternals).
Enver Gjokaj plays Officer Saunders (The Avengers) as well as SSR agent Daniel Sousa (Agent Carter).
Bridger Zadina plays bully E.J. (Iron Man 3 deleted scenes) as well as Mordis (Inhumans).
Tony Curran plays Bor (Thor: The Dark World) as well as Finn Cooley (Daredevil).
Matt Gerald plays White Power Dave (All Hail the King) as well as Melvin Potter/Gladiator (Daredevil).
Alfre Woodard plays Mariah Dillard (Luke Cage) as well as Miriam Sharpe (Captain America: Civil War).
Rob Mars plays an hospital orderly who tried restraining Thor (Thor) as well as muscle-for-hire Jerome Zandow (Agent Carter).
Tom Virtue played Thomas Richards, a Roxxon accountant executed by the Mandarin (Iron Man 3) as well as a motel clerk (Agents of S.H.I.E.L.D.).
Clancy Brown plays Surtur (Thor: Ragnarok) as well as Ray Schoonover/The Blacksmith (Daredevil).
There also are the people who cameo as different roles. (e.g. Stan Lee)
Outside of the MCU, there are plenty more, for example:
Ryan Reynolds plays Deadpool in the 2016 movie Deadpool, as well as playing whoever this is supposed to be in X-Men Origins: Wolverine: