First rule in government spending: why build one when you can have two at twice the price? https://www.youtube.com/watch?v=Et4sMJP9FmM

Honestly this is probably true sigh. the temptation is to say most of the cost of manufacturing an item is in r & d, setup, confirmation testing, etc these are mostly one time costs, the actual price of manufacturing the item is fairly trivial in comparison, this is what makes economy of scale work.

So one james web space telescope costs 10 billion and take 20 years to build. However two space telescopes cost 10 billion 100 million and take the same twenty years to build. three would be 10B 200M + 20Y etc.

But this would of only worked if you built the second one at the same time you had the setup and test facilitys prepared for building the first one, I will be generous and assume the r & d is not so space and time critical.

Personally I think the main argument for making a second is that you are putting this thing in a location where it cannot be serviced. The parable of having all your eggs in one basket comes to mind.

This burned into my brain when I was a teenager.