If you see a message stating that your phone number is already in use, it means the phone number is currently linked to an existing dua.com account. For security and account integrity reasons, the same phone number cannot be used on more than one account.
How to resolve this?
To add your phone number to your current profile, the existing(old) account linked to that number must be deleted first.
To proceed, please contact our support team by submitting a request. We will delete the old account associated with your phone number. Once this is done, you will be able to add the phone number to your new profile.