"Can anybody tell that why backup is getting started twice a time."
Rather than being a problem with cron this might be a problem with the way your computer keeps time. Have you told the BIOS to keep 12 hour time instead of 24 hour time?
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Be prepared. Create a LifeBoat CD.
http://users.rcn.com/srstites/LifeB....home.page.html
Steve Stites