Tautology

Tautology was the unnecessary repetition of words that effectively meant the same thing. The Sixth Doctor explained that "Galapagos" meant tortoise, and so the Galapagos tortoise was really the "tortoise tortoise." (AUDIO: Bloodtide)

Luke Rattigan was irritated by people who used tautology, particularly when the Tenth Doctor called his invention the "ATMOS system" when ATMOS already stood for "Atmospheric Omission System". This would be the equivalent of saying "Atmospheric Omission System system". The Doctor found Luke's reaction to the tautology insightful to the boy's personality. (TV: The Sontaran Stratagem)

The Fourth Doctor found the GlobeSphere company name a bit tautological, (AUDIO: Energy of the Daleks) while the Eighth Doctor later thought of the phrase "tidal wave" as pointlessly tautological since all waves are tidal. (AUDIO: The Next Life)

Professor Frederick Marius used the tautologies "at about eleven hundred hours this morning" and "a tiny little square of metal" in his computer diary, for which he chided himself. (PROSE: Tautology)

Behind the scenes
The title of the episode Deadly Assassin is considered by many to be a tautology as an assassin's job is to be deadly. However, Robert Holmes denied that the title was tautological, saying, "There are plenty of incompetent assassins." This redundancy was parodied in the spoof The Curse of Fatal Death.