Apart from using the PUSH method which sends a notification to your smartphone or tablet (recommended), you can also use a one-time passcode for Duo authentication in order logon to applications supporting 2FA. Passcodes are valid until they have been used. It is similar to the code in your security device for you to sign in internet banks. However, if a second passcode is generated and used before the first, then the first becomes invalid.
What is one-time passcode and when to use it?