What's being done with that infrastructure (clean room, testing facilities, vacuum chambers, etc..) right now?

Nothing?

Of course, the sensible thing to do is to build the test equipment, procedures, train the testers. Then test #1. Then test #2. Then test #3. That's what everybody does.

The cost of the Saturn V rocket was amortized over many launch vehicles, despite a host of new technologies that had to be developed for it, despite much of it being hand-built.

Other telescopes are being built, at least at places like Goddard SFC. Probably WFIRST aka Nancy Roman Space Telescope) there. Other facilities (Northrop Grumman) would be competing with NatSec/OGA instruments.

But the cost of grinding the mirrors?

Consider the cost of designing the mirrors, designing a machine to grind the mirrors, testing the machine, and verifying the grind of the mirrors.

All that adds to the cost of the first mirror, and adds $0 to the second.

But the grinding machine costs (which is significant if you have 10 of them) and the grinding process takes a lot of time and cost (if you have 1 machine x 10 mirrors).

I don't think these NASA/JPL guys are quite as dim as you seem to suggest (but you have more info) and they may have done a little cost/benefit analysis before embarking on the project.

From their statements, no, I don't think any cost/benefits of making a twin were done. In particular, none of the respondents wrote anything about what the cost of R+D was relative to the manufacturing cost.

BTW, I am not suggesting they are dim. But one can be the finest physicist in the world and not be familiar with cost accounting and manufacturing.