The developer's workflow of AGL includes a very convenient facility to put put developments in sandboxes.
Sandboxes can be used for trying, testing, forking, showing ideas, exchanging with others, ...
And sometime, when the development in the sandbox is achieved, the developer wants to push it in the master trunk. When the developer pushes it's sandbox branch in review he gets the message:
This is because the commit's messages have tags "Change-Id: XXXXXX..." already existing.