Each iPhone or iPad can only be associated with one Eyefi account at a time. If you log in to your Eyefi account on an Apple device once, you will need to remove it from your account before you can log in to the app with a different account. For instructions on removing a device from an Eyefi account, please see Remove a Device.
If you see the message "device already registered" when trying to log in to the Eyefi app, your iPhone or iPad is currently active on another Eyefi account.
If you do not know what account your iPhone or iPad is associated with, contact Eyefi Customer Care with the device’s Wi-Fi MAC address. You can find this MAC address by going to Settings > General > About > Wi-fi Address.