To add a second device, you need to be prompted for a Duo authentication. You may have the "remember me" option enabled on your preferred browser, so in order to force Duo to prompt you to authenticate, either use a private/incognito tab on your preferred browser, or use a browser you have not signed in to before.
1. Sign in to myRedDragon.
2. Before you authenticate with Duo, select the Settings menu in the upper right hand corner.
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3. Select Add a new device
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4. Before adding a new device, you will be prompted to authenticate using your current device
5. Select the device type you want to add and follow the prompts to enroll your new device.