Hey FunkyGrl,
We'd be happy to help out. Sounds like you're seeing more than one Messages app. While this is certainly unexpected, if you're certain this isn't another app that looks similar, we'd suggest restarting to get started.
If the issue persists afterward, make sure you're running the latest software, which would be iOS 16.0.2 for your iPhone 13, to rule out potential software issues. For help with backing up and updating, check out: Update your iPhone or iPad - Apple Support
If that doesn't solve the issue, hold down on the icon of the duplicate, tap Remove app and choose to remove it from your Home Screen. This option leaves the app installed and won't have any impact on your messages. If for any reason it removes both copies, just go to your App Library by swiping to the right and then hold on the Messages icon there and tell it to "Add to Home Screen" to get it back.
We hope this information helps out.
Take care!