If the R&D is done anyway, why not just wait and build a second telescope if the first one fails? I don't see why that would be any more expensive than building a backup ahead of time, and it means if the first one fails due to an undetected design flaw, they have a chance to correct it.
It’s cheaper to build two at the same time than shutting down production for years and trying to build another one. Tooling doesn’t last forever, and the process and skills required to build one can help you build the second one cheaper. But if you wait, that knowledge is often lost. For example, we have no way to build a Saturn V rocket today even having spent R&D and NRE decades ago.
Same reason NASA built the Space Shuttle Endeavour with spare parts left over from the others, instead of building it from scratch. People move on with their lives, and the facilities that manufacture these things do too. Assuming everything was well documented it should be possible to recreate the tooling and train new people, but that's much more expensive than building two in the first place.
