resend email

I create a schedule job and running at each 1,11,21,31,41,52 minute every hour. It should send email to receviers at the setted time. But, i accepted two emails of this report at the same time.  I have checked the scheduler, it just have one schedule job, and i also have checked the log, it just query dateBase one time. So, i don't know how to fix it. Has anyone  meet the same question? And please help.