In the rare occurrence that a message does not send properly the first time, you have the option to resend your message to the recipients who did not yet receive it.
Messages will attempt to send the message for one hour. In the case of a message not sending in that time, a "Failed" message will appear on the messages list, as well as on the Message Details screen, where you would normally see the sent timestamp. Hover over this message to see when the message stopped processing, as well as how many list subscribers did not receive it.
The green action button on the bottom-right menu will read "RESEND." Click the button and confirm that you want to resend the message. The system will attempt to deliver the message to the remaining subscribers. This should not result in duplicate messages.