By itself, an e-mail listener does not implement any synchronization and relies on the external mail server to delete processed e-mails to ensure they are not processed twice.

In a cluster, it is possible that under certain timing conditions the same e-mail will be downloaded and processed multiple times for a given task instance. However, this is not a problem, because during processing, the e-mail listener checks against a security GUID attached to the task. This is a one-time token; after a request is processed, the token is removed from the task, thus preventing a second e-mail request instance from being processed.