Without a complete and honest accounting of the risk profile, which is something NASA and its contractors have historically not been great at, it's hard to say

Without a complete and honest accounting of the risk profile, which is something NASA and its contractors have historically not been great at, it's hard to say whether test/backup hardware would have been warranted. 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.