As I mentioned towards the end of my look at The Mind Robber yesterday, the White Robot costumes were reused from a January 1967 episode of Out of the Unknown - "The Prophet". This came from a short story by Isaac Asimov, and was the last story of the second series.
It marks a tendency in TV production to reuse props and costumes as a way of making meagre budgets stretch further. This tradition continues to this day.
For example, the space capsule for The Ambassadors of Death was afforded by being shared between Doctor Who and another BBC production. Federation helmets from Blake's 7 are used in Frontios - and Blake's 7 makes use of a Sea Devil mask for the opening episode of its final series. Whilst the White Robots are a case of reusing something from another production - like the giant insects in Planet of Giants - there are numerous instances of props and costumes being reused within Doctor Who itself.
The most famous costume reuse is that of the Axon Monster. This red / orange costume was repainted in greenish hues for use as the bipedal form of the Krynoid in The Seeds of Doom. The costume designer was actually the same on both stories - Barbara Lane.
1979's Destiny of the Daleks required a variety of costumes for the slave-workers on Skaro. As you can see from this image, a Draconian outfit was reused, as well as an employee of the Sandminer from The Robots of Death.
The story also sees the reuse of a Mentiad costume from The Pirate Planet for another slave-worker, and a Morestran from The Planet of Evil. The helmets which Commander Scott and his troops wore in Earthshock reappear as part of the Bannermen costumes in the Sylvester McCoy story Delta and the Bannermen.
Earthshock also saw the introduction of new Cybermen - and new Cyberguns. The front section of one of these weapons appears later as part of Silurian technology in Warriors of the Deep. You can just see it in the foreground of this image.
When Roger Murray-Leach designed the Vogan sets for Revenge of the Cybermen, he devised a circular figure-of-eight emblem. 18 months later, he was called upon to design the Time Lord homeworld of Gallifrey for The Deadly Assassin. He elected to reuse the same emblem - and it is now known as the Seal of Rassilon.
To reconcile this duplication, some have postulated that the Time Lords were somehow involved in the history of Voga.
A more recent example of recycling is the yellow pod which the Doctor and Ida Scott use to descend into The Satan Pit. This reappears the following year as the central part of Professor Lazarus' genetic manipulation machine.
There are many, many more instances to look out for - like Abigail's cryogenic pod being used as a Cyberman cryogenic unit in Closing Time. Next time you settle down to watch a Doctor Who DVD, pay close attention to the small details - you may spot a lot that might seem familiar...