There are some doubles though. Voyager 1 & 2, Spirit and Opportunity…
I think the spy hubbles are better examples.
Can you imagine the added complexity for the Hubble design for it to work as space observatory or earth observatory.
I wonder if the he Hubble was myopic because of this.
IMO "build two, launch one and see what happens" was absolutely not the right approach here. But the goal of the program is to get a functional JWST instrument into position, and if there's any relatively (wrt. the entire program) cheap way they could have substantially de-risked that outcome—by tooling up for a possible second production run, flying a "dumb" article to test the deployment procedure in situ, or whatever else—it should have been seriously considered.
Frankly, this sort of judgment is something I don't trust NASA to get right in practice, through no major fault of their own. Congress runs NASA like they run the rest of the country: setting them up to fail, blaming them when they inevitably do, withholding realistic funding for congressionally mandated programs while simultaneously helping their military-industrial cronies wring out every last dollar. The only way to get anything at this scale done is to lowball up front and beg for more money later.
If everything goes as planned, having two observatories sure would be appreciated by the scientific community.
Also, I assume, most the costs are associated with the project and setup of fabrication.
To that I say that a single copy is the only reasonable option. JWT is essentially a prototype and a product in one. The first to employ such a complex radiation shield, The first to have berrylium mirrors (AFAIK), The first to orbit L2.
A copy would inherit any unknown problems with the first telescope, which is especially bad if you've read how many times the JWT has run overbudget and ran into issues. You're better off launching the prototype product to learn what will go wrong and go from there.
The lack of doubles in space shows how exiting the field is because I'd rather have novel prototypes than another hubble.