The best way to start troubleshooting issues of duplicate message delivery is review your Message logs. Each message that Twilio sends out is matched to a record in your logs, identified by a 34-character string starting with "SM" for SMS or "MM" for MMS (known as the Message "SID"). Check to see whether the 2 (or more) messages that the user received came from just 1 message record in your Twilio project, or if there are 2 or more message records.
If you have a lot of messages in your project, you can enter the phone number which the messages were sent to and then click Filter to only show these messages. You are going to be looking for a record for message in question, as well as other messages which are duplicates and which were sent within a few seconds of each other.
If you find one message
This indicates that Twilio sent just one message to the carrier. If your user received more than one, that means there was a duplicate with the recipient carrier, or with an intermediate party between the recipient carrier and Twilio.
Users of all types of phones do at times receive duplicate messages. It is typically the result of a safeguard carriers implement when they think there may have been a service disruption. Sometimes duplicate messages can also be produced by the destination carrier when the recipient is roaming.
Duplicate messages should be a rare occurrence, and if you see this happening for an extended period of time to several different recipient carriers, please let us know by collecting the message records of the messages that got duplicated and contacting support.
If you find duplicate messages sent within a few seconds of each other
This means that Twilio sent multiple messages to the carrier, and almost always means that Twilio received multiple POST requests from your app.
If you do see that duplicate messages were sent by Twilio, there may be a bug in your code that is making multiple POST requests to Twilio. Please check the HTTP request logs on your server as this may reveal more information about the requests sent to Twilio and will be a good place to begin investigating a possible bug.
Instances where Twilio's system has created a duplicate message without receiving a POST request from your application are extremely rare, but if you have checked your server logs and are positive that you only see a single request to Twilio, please tell us. It would be very helpful to our troubleshooting if you can send the SIDs of the duplicate messages to us via our "Talk to support" form.