Custom Cron hook invoked more than once within single cron-run().

Hi,
Need Help.
I have implemented HOOK CRON as "my_module_cron()" which does send mail notification to some sepecific user, regarding new content posted at site. Cron shcedule to run once per Hour.

When cron run, my custom "CRON" hook send mail, its should call once per cron. But it looks like it sending same mail more than once(e.g. for some node it send 2 OR 3 OR 4 repeated mails etc..).

I am sure, mail sending function not called from anyother place.

Thanks & Regards,