Not sure if this a known issue or not but I've been using Hangouts as my default SMS Messaging app and intermittently, when I send an SMS to my contacts, they're reporting that they're getting them twice. I've tested it out my texting myself and got the same problem. It's intermittent though as sometimes it'll just send one SMS. Hangouts is the only Messaging app that's installed. The recipients I'm sending to are using non Android devices (iPhones & Windows Phones).
I then downloaded another SMS app and switched over to that to find out if Hangouts is really the culprit and so far no one has reported that they've had 2 SMS messages from me. I've even text myself and got the SMS once. For now I've disabled Hangouts completely.