This video is about Google Two-Factor (Two-Step) but applied to DUO also since it is the same method.
What Does NNU's Two-Factor Authentication Protect?
NNU's Two-Factor Authentication protects your NNU accounts. Currently, Two-Factors protects your NNU account and your Google Apps account.
What Services DUO Protects
Canvas and related services (SSO)
Library Databases (SSO)
Various Internal Resources (SSO)
What Services DUO Does Not Protect
Google Apps (such as Gmail)
Campus Computer Login
Gmail is protected by Google's Two-Factor authentication system which is a separate system than DUO.
For more info see: https://duo.com/why-duo/why-two-factor-authentication-2fa