Arguable, I think. For one thing, they already _have_ duplicates for a
lot of the facilities (multiple bays in the VAB, for instance) dating
back to when budgets were more generous and it was expected that many
more flights would happen.

I'm not sure, however, that there aren't bottlenecks. You would also
need to increase the ground support staff to support two near
simultaneous launches. In addition, you will have to build more
spacecraft. If there is a mission that really only requires one
spacecraft, and you launch two, that either means you need twice as many
craft or you only conduct half as many missions.
Fourth, among all the fatalities in space missions, there has been
exactly one flight (Columbia) where having a second spacecraft flying
the same mission profile might have avoided the casualties. Given those
odds, flying two spacecraft for each mission is actually more risky.
(And it is not economically feasible to have a complete backup, say,
spreading 7 shuttle astros among two shuttles for a single mission.)

Until the early 70's it was routine to launch unmanned missions in
pairs. It allowed more flexibility in planning, and gave the chance of a
completed mission if one of the pair didn't work.
Not routine. There were occasions, but often the mission profiles
differed, and the spacecraft were not launched in close succession. The
major reason for launching in pairs was due to celestial mechanics. (If
you have an appropriate launch window only once a year or so...)
There have never been dual manned flights except when the mission was to
attempt rendezvous, but dual missions to the Moon were certainly
planned.

An would have required additional MSC control facilities, stressed the
DSN etc.