Sometimes NASA does send duplicates, including for Rovers. Spirit and Opportunity were identical. If all missions had a duplicate, we would have spent money to build an additional 15 spacecraft, and that would have only saved zero to two missions.

I'd guess it boils down to the relative expected contributions of random errors/failures vs. systematic errors/failures. Many failures were design flaws that would have created identical failures in any duplicate spacecraft of the same design. In that case, spending the money on two separate missions with unique hardware would have a higher return on investment than spending twice the money on one mission where either both spacecraft succeed and collect somewhat redundant data, or both fail and collect nothing.

Looks like NASA has attempted 28 missions to Mars, and four of these involved Rovers. All 4 Rovers have been successful. If NASA sent two rovers together each time, they'd simply have spent 1.5x money and completed the same # of missions.

Out of all 28 Mars missions NASA attempted, only 6 had some kind of failure:

- Mariner 3 and Mariner 4 were identical probes launched 23 days apart. Mariner 3 failed. We never figured out the root cause of Mariner 3 failure. Mariner 4 worked just fine.

- Mariner 8 and Mariner 9 were identical orbiters launched 21 days apart. Mariner 8 failed due to a botched installation of a diode onto a PCB which damaged that diode. Mariner 9 was successful.

- Mars observer failed. Root cause was never 100% definitively determined but NASA's final report focused on the hypothesis "that 2 grams of nitrogen tetroxide
slowly leaked through a valve during the 11 months the Mars Observer was in space. When the fuel system was pressurised, the nitrogen tetroxide mixed with monomethyl hydrazine – an explosive combination....If NASA’s managers had studied the design well enough to understand its limitations, they could have taken steps to prevent the failure by periodically purging the fuel line." It sounds like a duplicate would have had the same problem -- increased costs for no benefit.

- Mars Climate Orbiter failed due to a software bug. Any duplicate mission definitely would have had the same bug and failed in exactly the same way -- increased costs for no benefit.

- Mars Polar Lander failed due to a software bug. It's not 100% certain that the bug would have been triggered by a duplicate mission, but it's reasonably likely. "The Failure Review Board concluded that the most likely cause of the mishap was a software error that incorrectly identified vibrations, caused by the deployment of the stowed legs, as surface touchdown. The resulting action by the spacecraft was the shutdown of the descent engines, while still likely 40 meters above the surface. Although it was known that leg deployment could create the false indication, the software's design instructions did not account for that eventuality."[1] I'd personally chalk a duplicate up to increased costs for no benefit, but an expert's interpretation may vary.

- Deep Space 2 failed, no root cause identified. 2 possibilities identified were design flaws, which could have affected a duplicate as well. 4 possibilities involved bad luck in landing so it's possible a duplicate would have had better luck.

So, if all missions had a duplicate, we would have spent money to build an additional 15 spacecraft, and that would have only saved zero to two missions.

[0] https://www.newscientist.com/article/mg14119080-300-cheap-de...

[1] https://en.wikipedia.org/wiki/Mars_Polar_Lander