Messaging app keeps messing up recipient?

Hi, for some people, when I go to message them, where it shows their # under their name at the top of the messaging app, the number is wrong. It shows a 20 digit # instead of a 10 digit one, with the 1st 10 digits being my # prepended to the recipient's number. This prevents texts from going through. Every time, I have to click on it and delete the 1st 10 #s. Anyone else have this issue and know how to fix it?