Concurrency check, please? Running 2 at once = permanent deadlock

[ClaireCJS]

I wouldn't ask for one, but I am dealing with a queue of jobs, and about 1 in 50 times, 2 get run at once.
Of course this shouldn't happen—but it did.

Neither ever finishes.
They deadlock each other.
There's no way out other than ^C and starting over.

Manual intervention is what I'm trying to avoid, so could it just "notice" the other one, and not deadlock like that please?