With "commit already exists (as current patchset)" or
"commit already exists (in the change)" error message
Gerrit rejects to push a commit to an existing change via
refs/changes/n
if the commit was already successfully
pushed to the change.
With "commit already exists (in the project)" error message
Gerrit rejects to push a commit to an existing change via
refs/changes/n
if the commit was already successfully
pushed to a change in project scope.
In any above case there is no new commit and consequently there is nothing for Gerrit to do.
For further information about how to resolve this error, please refer to no new changes.
Part of Gerrit Error Messages