I will never understand why NASA doesn't build at least two copies of expensive space telescopes like this. One single launch failure and boom (literally), the entire project is a failure?

It wouldn't double the cost assuming they do it with even a modicum of efficiency and intelligence. And hey - assuming that the launch goes smoothly, you can get double the benefit

I worked briefly in a satellite lab, and we did indeed have duplicate parts for most things.

Any space telescope with a mirror, is a different story, e.g.

https://science.nasa.gov/mission/webb/webbs-mirrors/

The RST has an 8ft mirror. The mirrors have to be ground and polished in a clean room to tolerances that are the highest in the world. I vaguely recall it's a square law or worse - for radius-to-time-to-polish. So you're paying for time, and you're paying for the expertise of one or two custom built places that can manufacture them.

Because the incentives are not properly aligned. The US has historically wagered big money on singular missions, which given the cost per launch makes perfect sense. Now that costs per launch are decreasing rapidly, and critically launch availability is also increasing, the opportunity for scale is presenting itself.

why build one when you can build two for twice the price?

I guess for one thing, these launches don't tend to fail and second, they actually do make duplicate missions sometimes. See Voyagers 1 and 2, Spirit and Opportunity and Curiosity and Perseverance.

They have often indeed made “flight spare” copies of the main instruments.

When I worked for Mars Global Surveyor, one of the museum-pieces in the lobby was a glass case with the Viking spectrometer flight spare.

The flight spares were exceedingly useful when the spacecraft had some kind of trouble during its mission, because you could mock-up the flight spare in a lab and replicate many situations very closely, and propose solutions based on the mockups.

Whatever the reason, it seems specific to the intersection of space and telescope. Space probes, landers, and crewed vehicles all get made in multiples, as well as ground based telescopes like Keck or the gravitational wave observatories.

what if both of them blow up?