The Doctor's spare TARDIS key

At various points in their life, the Doctor possessed one or more spare TARDIS keys to the one that was primarily owned by them.

Occasionally, the Doctor was able to give out additional keys to their companions, including Susan Foreman, (TV: The Sensorites) Steven Taylor, (PROSE: The Empire of Glass) Sarah Jane Smith, (PROSE: Interference - Book Two) Tegan Jovanka, Nyssa, (AUDIO: Hexagora) Peri Brown, (PROSE: Fascination) Erimem, (AUDIO: Three's a Crowd) Anji Kapoor, (PROSE: The Crooked World) Charlotte Pollard, (AUDIO: Time Works) Lucie Miller, (AUDIO: Relative Dimensions) Rose Tyler, (TV: Aliens of London) Jack Harkness, (TV: Bad Wolf) Martha Jones, (TV: 42) Donna Noble, (TV: The Poison Sky) Amy Pond, (PROSE: Nothing O'Clock) Clara Oswin Oswald, (TV: The Snowmen) Clara Oswald, (TV: The Time of the Doctor) River Song, (TV: The Husbands of River Song) and Bill Potts. (PROSE: Bill and the Three Jackets)

History
During his adventures alone with his granddaughter Susan, the First Doctor did not possess a spare key. When he destroyed Susan's key he told her that he could make another one at a later date. (TV: "The Ordeal")

The TARDIS and the TARDIS key were taken by Marco Polo who wanted to gift it to Kublai Khan in return for his freedom from his court. To gain access without Polo's knowledge the Doctor constructed a second TARDIS key, although it was also confiscated when Polo learnt of its existence. When Polo with the Doctor and his companions completed the journey to Peking and Polo gained his freedom without needing the TARDIS it was returned, along with both keys, to the Doctor. (TV: Marco Polo)

The Doctor later gave Susan this key as a replacement for the one he destroyed. (TV: The Sensorites) By the time he reencountered and had his key stolen, the Doctor did not have a spare key, forcing him to find Susan and use hers, only for the Master to beat them to the TARDIS. (AUDIO: The Destination Wars) As it was not returned to him when Susan left the TARDIS, (TV: The Dalek Invasion of the Earth) the Doctor was left without a spare key for some time afterwards. (PROSE: The Plotters, The Empire of Glass)

Later, a spare key fell out of the Doctor's cloak and Ben Jackson picked it up. When witnessing the Doctor entering the TARDIS, Ben was reminded he possessed the spare key and persuaded by Polly Wright to return it. Unlocking the TARDIS with their key in pursuit intending to return it the duo entered starting their adventures with the Doctor. (TV: The War Machines) Ben and Polly were not able to return the key immediately as their attention was soon drawn by the TARDIS' space-time capabilities. (TV: The Smugglers)

During the time of the Fourth Doctor, the spare TARDIS key was found in a cubbyhole above the "X" on the police box exterior. River Song used this key once to "borrow" the Doctor's ship. (AUDIO: Whodunnit?)

At least early in his life, the Fifth Doctor kept a spare key on his person. At one point he gave the key to Tegan Jovanka in case the party was to get separated. Whilst under the influence of Monarch, Adric briefly struggled for possession of the key but Tegan escaped. (TV: Four to Doomsday)

As early as his travels with Ace and Mel, (AUDIO: The High Price of Parking) the Seventh Doctor kept a spare key in the cubbyhole above the "P" outside the TARDIS, (AUDIO: The Architects of History) although later in his life it had gone. (AUDIO: Survival of the Fittest) By the time of his regeneration into his eighth incarnation it had been restored and was used by the Eighth Doctor when Chang Lee had taken the TARDIS key from his possessions. (TV: Doctor Who)

The Tenth Doctor kept a second key on a cord of twine. It was stolen by the Weeping Angels when he was transported to 1969. It was recovered by Sally Sparrow who also returned the TARDIS to the Doctor. It stayed in her possession for another year. When Sally met the Doctor in person she gave him a file of documents, which the spare key was likely contained within. (TV: Blink)

The Twelfth Doctor possessed seven TARDIS keys of which Clara Oswald was aware. She threw every one of these into lava, destroying them, in an attempt to force the Doctor to save Danny Pink from death. After she had done this, the Doctor revealed the scenario to be a dream, rendering her actions ineffective. (TV: Dark Water)