We have some more information about this issue in our Duplicate text message (SMS) or picture message (MMS) sent or received multiple times: Android document. This issue wouldn't impact your billing. One of the biggest causes of this is the device freezing up when sending or receiving the text / picture messages. I'd recommend following the Device is slow / freezes / restarts / or displays memory errors: Android document to ensure the device isn't freezing and reduce the duplication as much as possible.
HTC has identified that this issue happens more frequently on this device than other devices and we are working with them to get that resolved as quickly as possible.
Thank you for your patience!