This is in fact done (effectively). All components have a "flight spare", which is just a second copy. It can be swapped should any component fail before take-o

This is in fact done (effectively). All components have a "flight spare", which is just a second copy. It can be swapped should any component fail before take-off. It doesn't get assembled into the whole thing though. Other uses include debugging already-launched kit with the easily accessible copy, reusing in other projects, and ultimately, museum artifacts. It is also worth saying that producing anything, from mirrors and cameras, to boring screws and nuts, that end up going into space, is no small endeavour. It's not the case that you can just mill something out of aluminium, then do it N more times. Here's a nice video about it: https://www.youtube.com/watch?v=QlASewYMDsg https://www.youtube.com/watch?v=QlASewYMDsg (in fact in the specific context of JWST)