freinds the issue is with the notifications setting "View in Lock Screen" & alert type "Banner"
- If you turn this setting "OFF" the alert doubling stops permenantly.
-OR You can either choose none or alert instead of banner in notification option.
I found this double msg alert in every other app too,like yahoo,whatsapp,imsg,message,app deals,pimp my screen.
It is the issue of how user want message alert in phone,selecting banner or view in lock screen creates problem.
Any one have any permanent solution to fix it apart from these tricks.