Each Android phone or tablet can only be associated with one Eyefi account at a time. If you login to your Eyefi account on an Android device once, you will need to remove it from your account before you can login to Eyefi with a different account.
If you receive a message "device already registered" when trying to log in to the Eyefi Android app, that means this device is currently active on another Eyefi account.
If you do not know what account your Android device is associated with, contact Eyefi Customer Care with the device's WiFi MAC address. You can find your Android device's MAC address by going to Settings > About phone/tablet > Status. Scroll down to see the Wi-Fi MAC address.