If you get a login request you weren't expecting, press Deny to reject the request. You'll be given the ability to report it as fraudulent, or you can tap 'It was a mistake' to deny the request without reporting it.
If you have notifications turned on for your mobile device, you may be able to respond directly from the notification. This WILL NOT WORK if notifications are turned off on your mobile device. You will have to open the DUO Mobile App to approve.