Skydivers normally wear a backup parachute. Apollo 15 survived with two out of three parachutes working properly. But wouldn't it be great to have a second deployment option, like skydivers do? The deployment maneuver itself must be a huge part of the risks with parachute landings from space. AFAIK reserve parachutes have never been used in space travel. Is the mass penalty of an independent secondary parachute system so big that it is easier to instead make sure it works every time?
Answer: Redundant chutes are used rather than back-up chutes because (for a given safety factor) they are lighter.
As an example, let’s approximate that both the weight and the effectiveness of chutes are proportional to the total surface area.
Let’s assume NASA is willing to accept a 1:10,000 mission failure rate and the failure rate of an individual chute is 1:100.
Design Team A recommends a single chute, with a back-up chute. Each chute weighs 100Kg, for a total of 200Kg
Design Team B recommends 3 chutes, but no back-up. Each chute has half the area (and half the weight) of Team A’s chute, for a total of 150Kg
Both designs have a mission failure rate of 1:10,000. But Team B’s design is 25% lighter. Who gets the contract?
There are many factors which complicate this simplistic analysis. A failed chute could foul the deployment of the backup. Multiple deployments at the same time may increase the failure rate, etc.
Backup parachutes have been used: the Mercury capsules had both a main chute and a reserve chute, and the reserve would only be deployed if the main didn't work right. Almost* every capsule since then used N+1 redundancy instead, where the capsule can land with one failed main parachute. Since capsules from Apollo onward were too heavy to land under a single parachute, this gives a reasonable safety margin without the weight penalty of a full set of spares.
* Gemini didn't have any redundancy for the parachute. Instead, if the main parachute failed, the crew would eject from the capsule.