Why is the first thing people suggest always "flash your phone"?!

Check first if the weirdness is for a reason. Do multiple contacts have the same number or something like that?

If you want to go with the wiping/flashing route, first try doing just a wipe of SMS messages by long-holding in Messages and doing "Delete all messages".

If that doesn't fix things, in Settings, Reset you can "Clear device", which is as good as a reflash for non-bricked phones, but is easier.