Barney Miller recycled more actors than any series I can recall. I couldn't remember more than a couple specific examples but, thanks to IMDB, here are a few who played multiple characters:
JOHN DULLAGHAN: Jackson the Pickpocket, Sylvester White, Harold Durrell, Milt Loftus, Ray Brewer
STANLEY BROCK: Arnie the Club Owner, Burgess the Barber, Bruno Bender
DON CALFA: Mr. DiLucca, Angelo Dodi, Leon Bidell, Calvin J. Kendall, Gilbert Lesco, Arthur Thompson, Edward Pratken
PHIL LEEDS: Arthur Bloom, Lou Hector, Harry Kruger, Horace Chandler, Gilbert Letier, Louis Nash, Brent LaMear
PHILIP STERLING: Mr. Buckholtz, Noel Cadey, Mitchell Warner, Judge Philip Paul Gibson, Frank Rilling, Howard Spangler
PEGGY POPE: Lonna Lane, Frances Newbound, Lois Marcum, Doris Carlisle, Ruth Bergendahl
KENNETH TIGAR: Fletcher, Stefan Koepeknie, Elliot Porter, Jesus Christ, Philip Pollock
FLORENCE HALOP: Mable Kleiner, Mrs. Pierce, Evelyn Holly, Karen Golden, Wanda LaMear, Wilma Kestner
Hell, even George Murdock, who played Lt. Ben Scanlon from 1976 to 1982, played another character (Master Sgt. J.R. Reville) in an episode from 1977. Maybe they didn't know, at that time, that Scanlon would be a recurring character.