Help What should we store as a unique Id in the 3rd party database when registered to GCM

[Android question]

The registeration Id which GCM provides on registering the android device, often changes when we re-install the app. In order to avoid duplicacy in 3rd party database, what should we use as unique key in the database, so that using this field I can update the old registration Id with new registration Id whenever app is reinstalled.