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Yes. Several times. Among the actors who have played two or more different parts are (in no particular order): David Tennant, Karen Gillan, Peter Purves, Nicholas Courtney, Eve Myles, Patrick Troughton, Jean Marsh & Colin Baker. There are many others. If appearances in Torchwood and The Sarah Jane Adventures were also to be taken into account, there would be even more.

Several actors have appeared as different characters in Doctor Who. Putting aside cases where the there are plot reasons for this (e.g. Stories like "The Enemy of the World", "Arc of Infinity"):

-- Jacqueline Hill played Barbara, a companion of the First Doctor. Hill later played a priestess in the serial "Meglos"

-- Jean Marsh played Sara Kingdom in "The Daleks' Master Plan"; she later played Morgaine in "Battlefield"

-- Nicholas Courtney played Bret Vyon in "The Daleks' Master Plan"; he later played semi-regular character Brigadier Lethbridge-Stewart.

-- Eileen Way played the Old Mother in the first serial, "An Unearthly Child"; she later played Karela in the serial "The Creature from the Pit".

-- Colin Baker played Gallifreyan guard captain Maxil in "Arc of Infinity"; He later went on to play the Sixth Doctor.

-- Peter Purves played an American tourist in the serial "The Chase"; he later appeared as Steven in the same story, and became a companion to the First Doctor then.

-- Sarah Sutton played the Doctor's Trakenite companion Nyssa from "The Keeper of Traken" to "Terminus"; she appeared during that time as Lady Ann Talbot in "Black Orchid".

Freema Agyeman and Karen Gillan in the new series played other characters (Martha's Cousin, and a Soothsayer) before going on to play Martha and Amy.