A good practice when using two-factor authentication is to print out backup codes in case your primary factor such as cell phone is unavailable. These codes can be stored in wallets or other secure locations.
1) Send backup codes to your cell phone
Follow Using Duo with Any Cell Phone or Landline to send a SMS/text message contain your backup codes. Each code can only be used once and Duo will track and provide you with a hint of the next code to use. If you request another round of codes, the previous ones will be invalid.
If you have the Duo prompt to automatically set for push notifications, you need to cancel that first before selecting Enter Password code to send text codes.
2) Print backup codes
Once you receive the backup codes in a text message, use the normal procedures on your device to print the codes. If printing is not an option, write the codes on a piece of paper. Store codes in a secure location such as wallet. Do not store written password and codes in the same location.