Thanks for posting. The answer is that you want to cram in as much volume as possible without compromising recovery. Running 7 days a week will always result in better performance if you can adequately recover. Whether genetically or situationally, elite and pro runners can get away with this, or at least schedule their lives around running 80+ miles a week and don’t need that day off.
When we write the plans, we assume a “typical” athlete who probably needs a day off, not just physically, but physiologically as well.
Volume + Frequency = performance, except when you can’t recovery adequately. Your proposal to double up is a reasonable compromise: you’re getting all the volume and still getting a full day of recovery in there.
So the answer is: it depends. Frequency sort of wins out if you can run 7 days a week plus some double days, but if you can’t, then doubling up is an option to free up a full day. But, what if too many double days actually make your recovery worse? Some athletes are better off with no days off than compressing too much into too short of a period (you can take this though experiment and extend it to running 60 miles one day a week with 6 days rest instead of splitting up that 60 mile over 7 days: at some point, extra days of recovery makes things worse).