Update - not a fix to the actual problem, but a workaround:
installed the package "Shellcmd". renamed the script without the ".sh" extension. added this into "Shellcmd: settings" : /usr/local/etc/rc.d/send_gmail_after_startupNow it only runs once as required.
same goes for any other custom scripts needed to run once at startup.Hope that helps anyone else experiencing the same problem.