Just don't break your long run day into doubles!
....
Let's take a step back because you have to evaluate doubles vs single in how they fit into the bigger picture.
Every run and also training phases have a purpose.
Long runs are for building that endurance that helps you stay strong in the last half of a race.
Some workouts are for getting good a running fast.
and easy days are meant for recovering.
Volume phases are meant to make you efficent and a better runner, building all of the physiological adaptions that runners need.
This applies to doubling by
a) You can use doubles to keep mileage higher while still recovering by breaking your recovery days into two runs.
b) or use them to get an extra easy run before a workout, again really using them for recovery.
c) or just to run more because even if you're not building endurance from running 45+ minutes, you're still practicing and getting better at running.
Lastly,
on some days the benefits of a schedule 4+48
See, talk of doubles can't generalized, but it's moronic to say only to run in singles. That's just a narrow minded and inflexible.