Sending duplicate messages can be a real nuisance for you; it could be costing you a fortune, and it can be equally annoying for your friends, who know that when they receive one message from you then an identical one is sure to follow soon. There could be a number of problems causing this issue, depending on your model of phone or the strength of the signal provided by your service provider, but there are a few things you can do to try to remedy the issue.
Ensure that you have a good signal before attempting to send the message. Sometimes, if you lose the signal midway through sending a message, your phone will send a duplicate (or tell you the message has not been sent, and prompt you to send a duplicate) even though some of the message, or perhaps the whole thing, may have sent.
Take your phone handset to be checked. If the problem is not related to the signal quality, then it may suggest a problem with the hardware (your phone). Taking it to an expert will give you a better idea of the cause of the problem if it persists.
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Visit your phone manufacturer's website, and check for a firmware update. If it is a known problem with your product, then the chances are they will be working on a fix -- which may already be out and ready for you to download.
Remove the battery from the back of your phone, and leave it out for about a minute before replacing it. This has been known to remedy the looping issue -- but it could return if it is a hardware or service carrier problem.