We treat returning guests a little bit differently, as we assume you have already developed a relationship with them. For example, some automated responses might be redundant (and sound unnatural) to them since they have already received the response before.
For that reason, by default, we do not send booking confirmation to returning guests (on Airbnb).
However, if you want to send booking confirmation messages (triggered by the "New reservation" messaging rule) to all your guests, you can activate it in the account settings.
To send booking confirmation messages to returning guests:
Go to Settings.
Click on Preferences.
Click on Guest experience on the sidebar.
Use the toggle to activate Send booking confirmation to returning guests.
Don't forget to Save changes.