So is this the first time you've received that 2 notification but there's only one message? Or has it happened before, and if so, can you recall if it was sent from the same person?
The Textra app having a pretty large feature set with lots of customization options, it might be worth looking in its Settings menu in the Notifications section to see if any option might be relevant to what happened.
If this is the first time it happened though, it could just be a one-time glitch too. Or you might want to send a text message to whomever sent you this message, explaining that two message notification, and ask them to re-send that exact same message to see if the same thing happens on your end.