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just in case, check again
Do you have any other messaging app?
Boot on safe mode, if you don't encounter this issue then its an app acting up
http://talk.sonymobile.com/t5/FAQ/How-to-boot-your-phone-into-safe-mode/m-p/348008#U348008
Yes, it could be a messaging app, have about hangouts, have you combined SMS w/ hangouts? any notification app?