Duplicate txt msgs on iPhone

I'm getting duplicate text messages on my iPhone and AT&T is charging me for these duplicate messages. It normally happens if I receive a msg but don't unlock the phone to read the msg. I'll get a duplicate notification 3-10 mins after the 1st notification. However, I just received a msg, unlocked the phone to read msg, then received another notification a few mins later, but not msg was showing as new. I tested this on my mom's iphone and she said she also received a duplicate notification when she didn't unlock the phone to read msg. I've had Apple look at the phone and updated software. They said they've never heard of this problem and told me to dump everything on the phone and reload back to factory setting but will this delete everything in iCal on laptop? Anyone else notice this issue with their iphone?

Thanks in advance!