If you want to show a notification only to new visitors, or returning visitors, just follow this guide:
Edit your notification
Go to the "Customize" step > "Display Rules"
Select the relevant option for "Show this notification to"
The available options are:
All: show the notification to everyone (default).
New: the notification will be shown to new visitors only (visiting your website for the first time).
Returning: the notification will be shown to returning visitors that already visited your website at least once.