Under some circumtances some problem come from modem. For confirming modem fault or not, try use some 3g dongle as sms gateway. Preferable is huawei ones. If the huawei dongle not happening double sms. Then your modem have some firmware problem. If still occur need to investigate it is software problem or operator problem.
If operator problem normally its a pending sms that send on earlier. That also we experienced before