With this error message Gerrit rejects to push a commit if its commit message contains a Change-Id for which multiple changes can be found in the project.
This error means that there is an inconsistency in Gerrit since for one project there are multiple changes that have the same Change-Id. Every change is expected to have an unique Change-Id.
Since this error should never occur in practice, you should inform your Gerrit administrator if you hit this problem and/or open a Gerrit issue.
In any case to not be blocked with your work, you can simply create a new Change-Id for your commit and then push it as new change to Gerrit. How to exchange the Change-Id in the commit message of your commit is explained here.
Part of Gerrit Error Messages
