This is most likely an issues with the Viber's servers because this shouldn't happen. On a normal usage situation, that notification only pop up one time, then the contact will appear as a normal contact. If you have issues, then I recommend you reinstall Viber and update it to the latest version and the error should disappear completely.
Based on your version, access the official website to install the latest version: http://www.viber.com/en/