This may happen if you clear your app data without signing out first, as the cloud service will recognise your device as a new installation. To remove duplicate devices, tap and hold on the item in the device list to remove it from the cloud. This will permanently delete the information from your old installation and restore your device quota.
To make it easier to identify inactive device listings, you can compare the 4-digit identifier code to the right of the device name to distinguish between two device listings with the same name.