I’m assuming you’re using the Guest Smart Locks smartapp that comes with SmartThings. That smartapp sends the notification (if you’re installed the Classic and new mobile app, you’ll get 2 notification, one from each installed mobile app - the SmartApp sends one notification but each mobile app delivers it).
Unfortunately there’s no way to turn it off. If you have access to RBoyApps, check out the LUM app which allows you to customization notifications for each user / lock etc: [OBSOLETE] Lock User Management (LUM)
If you want only one notification, then turn off notification in your mobile settings for the second ST app.