(written from a Production point of view)
Robin Adair MacKenzie Brown (born 17 November 1926; age 88), known as Robert Brown, is an actor who has made one appearance on Star Trek: The Original Series, playing both Lazarus-A and Lazarus-B in the episode "The Alternative Factor". He was hired as a last-minute replacement for actor John Drew Barrymore, who had been cast in the role but didn't show up on the first day of shooting. Coincidentally, his first filming day on the episode was his 40th birthday.
Brown filmed his scenes for the episode on Thursday 17 November 1966, Friday 18 November 1966, Monday 21 November 1966 at Desilu Stage 9, Tuesday 22 November 1966, Wednesday 23 November 1966 at Vasquez Rocks, and Friday 25 November 1966 at Desilu Stage 10. 
Before Star Trek, Brown shot a pilot with William Shatner about a Swede and an Irishman who emigrate to America. Brown played the Irishman. It aired but was not picked up as a series. Another of Brown's pre-Trek jobs with Shatner was in a 1963 episode of The Dick Powell Theatre entitled "Colossus". That episode also featured Frank Overton. It was Shatner who recommended Brown to replace Barrymore. He was literally dragged to the set without any rehearsal.
He went on to star in the TV series Here Come the Brides from 1968 through 1970. This series also starred TOS guest actors Mark Lenard and David Soul. That series featured a large number of Trek actors in guest-starring roles. The episode "A Kiss Just For You", for example, featured Michael Forest, Kathryn Hays, Gary Pillar and Ralph Maurer. Ishmael, a popular 1991 Star Trek novel by Barbara Hambly, featured a crossover with Here Come the Brides.
Robert Brown's last known acting role was as a guest star in a 1994 episode of In the Heat of Night directed by Reza Badiyi.
Trivia: Robert Lansing, whose real name was Robert Brown, was forbidden from using that name by the Screen Actors Guild because Brown was already using it.