Ensure that the same process never appears twice in the list of process_to_run.
Otherwise, it would be awaken twice. The first time, it would execute then
wait on the completion of a new simcall. The second time it would appear
to return from the new simcall but the simcall would not have been
executed.
Since resume and kill are particular functions/simcalls that change
the behavior of other process, it is ok if the process was already in the
process_to_run list (although it must not appear twice). For other simcalls,
I think it is invalid if two simcalls lead the scheduling of the same
process.
