This bug concerns publication through a repository gateway.
In this case of a transaction submitted with a generic tag, which contains a timestamp, the cvmfs_receiver process performing the transaction commit will update the timestamp to actual time when the commit is being done. This way ensures that the timestamp reflects a useful time, in the case when the transaction was waiting a long time due to a busy gateway and is a way to prevent timestamp collisions when two publishers submit a transaction at the same time.
With multiple short transaction commits (multiple per second), the "cvmfs_receiver" process may attempt to create the same generic tag, leading to an error.
The simplest solution is to just wait a second in the case of a collision and update the timestamp in the tag.