This happens when you try to add a device with the exact same DL ID, or you are trying to use the same name as a device already saved in the list. Every device must contain a unique name. When you are adding a new device, security sensor or home control device, you must change the name to a "Custom Name" or to one of the other quick naming options. If you try to add device with the exact same name, you will get the message "Device Already Registered".
Press the Device Name drop down arrow, and change to Custom Name or one of the other quick naming.
This comes up mostly with adding a second Keyfob or CO Detector.