The support structure is also a large cost in solar power farms. So this would not double cost. Also, the backside will not need as much cleaning
But here near the equator where view of the sky is the only thing that matters, they are a non-starter. Perhaps in solar farms where panel mounting is 4m or higher?
But no, at 20%, I don't believe anybody will want this. Maybe in a decade that changes, but not right now.
2) You've assumed the panel costs double, which is likely false.
3) solar generation costs have been falling exponentially and upgrades are incorporated into various installations and new developments all the time.