> An easy way to cover that is to have the script create a file, which you check at the start of the script and if present you exit.

This does sound a good solution on paper, but "checking at the start" and "creating a file" are two different steps (i.e. not atomic) and will cause trouble eventually if your system has the tendency to run the script twice. A better solution is to use the `flock` command before creating the flag file. For example,

  exec 999>/var/lock/my.lock
  flock -n 999 || exit 1
  if [ ! -f flag_file ]; then
    echo "script not ran before, running"
    touch flag_file
  else
    echo " already ran, exiting"
    exit 1
  fi
  # do stuff here