Spare Body Parts

Shepard: [The Normandy] was destroyed in a Collector surprise attack. I ended up spaced.
Wrex: Well, you look good. Ah, the benefits of a redundant nervous system.
Shepard: Yeah, humans don't have that.
Wrex: Oh. It must have been painful, then.

—Mass Effect 2

Never trust something with an odd number of limbs.

—Catharsis

A Subtrope of Bizarre Alien Biology. Humans have two eyes, two legs, two arms, two lungs, one mouth, etc., as do most of our most familiar animals. Want to make a race seem alien? Give them extra of something.

Extra Eyes and Multi-Armed and Dangerous are subtropes of this.

Examples of Spare Body Parts include:

Comic Books

Spiral of Marvel Comics has six arms, though the extra pairs are robotic additions made by Mojo. Fittingly, one of her aliases is "Shiva".

Film

A pianist from Gattaca was genetically engineered to have twelve fingers. There are apparently enough people like this around that some pieces of piano music require extra fingers to play.
A prostitute in Total Recall has to wear a special kind of brassiere...

Literature

The Hitchhiker's Guide to the Galaxy's Zaphod Beeblebrox: Two heads, three arms.
The CoDominium Moties have one large, powerful hand on their left side, and two smaller, more dexterous hands on their right side.
S.L. Viehl uses this a lot in the Stardoc series. For example, there's an alien in the first book with two separate digestive tracts.

Live Action TV

The lounge singers in the original Battlestar Galactica had two mouths each. Meanwhile those insectoid things had four arms. Eek, in both cases.
Star Trek‍'‍s Klingons have a number of physiological redundancies such as an extra liver, a double set of ribs, redundant nerve systems and an extra lung.
Centauri in Babylon 5 have two hearts. Unlike Klingons, Centauri can die if either heart fails.
- In the season five episode "The Very Long Night of Londo Mollari", in which Londo suffers a heart attack, we find out more details: the right heart is a large and simple muscle, much like a human heart, whereas the left heart is a knot of blood vessels that gives the blood pressure an extra kick and also functions much as a human kidney (mollusks on earth have a somewhat similar system). Surgery on the left heart is correspondingly difficult due to its complexity.
- Males also have six prehensile penises located on the sides of their torsos.
Doctor Who: Time Lords have two hearts. The Doctor is no exception.
- Some of the spinoff novels have added a great deal of lore regarding Time Lord hearts, including:
  - Time Lords have only one heart in their first incarnation and the second one develops, along with other physiological advantages like the respiratory bypass system, as a result of their first regeneration.
  - The hearts are categorised as a "primary" and a "secondary". Quite what this means in a physiological sense (e.g., is one heart more powerful than the other, do they have somewhat different functions) is never made quite clear.
  - The Eighth Doctor novel "The Adventuress of Henrietta Street" suggests that the second heart might be the source of some sort of psychic link between the Time Lords and Gallifrey, and the control centre for their various alien traits (regeneration, rapid healing, sensitivity to time etc).
- Both the spinoff novels and the new series indicate that the Doctor is capable of living with only one functioning heart, though this weakens him.

Religion

The Hindu deities Kali and Vishnu have two pairs of arms (and the former can upgrade to five), and Surya is sometimes depicted with four arms.
A specific Buddha exists who literally has a thousand eyes (all the better to see with) and an equal number of arms (the ultimate multi-tasker).
- Very likely borrowed from Hindu mythology, where extra heads and limbs tend to be the norm.

Role Playing Games

Gamma World. The mutant race known as the orlen were humanoids with two heads and four arms. Additionally, players could create mutant characters with any number of extra parts.

Tabletop Games

In Warhammer 40,000, the Genestealer Hybrids, an Alienesque cult designed by the equally Alienesque Tyranids, have six limbs, four of which are the normal mammal-human ones, and the two (or sometimes only one) others are clawed and chitin-ed.
- For that matter, the Space Marines have 19 extra organs implanted.

Video Games

In Mass Effect, the batarians have four eyes. When humans, or other two-eyed races have conversations with them they get confused because they don't know what to focus on, leading to the batarian attitude that humans are stupid.
- And speaking of Mass Effect, there's the memorable quote about krogan testicles: "Some go for as much as 10,000 credits a piece, that's 40,000 for the full set."
- Mass Effect actually takes this trope to its logical conclusion with the krogan, who have evolved redundant internal organs as a result of living on a Death World. At one point EDI mentions a krogan's tertiary organ systems, meaning even their spares have spares. And when one of the redundant ones kick in, it triggers an intense adrenaline surge that permits them to knock people aside like ragdolls. Never attack a krogan with anything less what you'd use to demolish a building. You'll just make him mad.
Gilgamesh from the Final Fantasy series had at least eight arms.
The Shokan (the race of Goro and Sheeva) in Mortal Kombat all have four arms.

Web Comics

Dwarves in Order of the Stick have two livers.

Western Animation

In an episode of Futurama, Bender was in an accident that cost him the use of his arms. Beck gave him tiny replacement arms mounted on his neck (which Beck used to play the harmonica). Later when he regained the use of his original arms, he'd use both pairs in expressions.
- Zoidberg as well. He's got several of everything, and can even talk through a vivisection (much to the dismay of vivisectors).
Men in Black features the Sintillians. Not only do they have two hearts, but neither heart ever stops working; so long as they have at least one and "nobody drops a piano on them", they're effectively immortal. When Alpha steals a Sintillian's heart, K makes it clear to J that the Organ Theft is still a big deal. "You have ten toes. You wake up one morning with one missing, how would you feel?"