Add option to hash plugin to hash multiple fields.

I am using Drupal 7.12 and the latest release of Feeds Tamper. I am using a standard setup for feeds fetcher, parser and processor using Feeds Import 7.x-2.0-alpha4. I've set up a mapping for a GUID field where I have configured the Feeds Tamper to calculate a hash on this field. My understanding is that the hash plugin will serialize the record in the feeds upload and save that as a unique identifier for this record. However, it is only updating a single record regardless of the content, not creating a new node. If I ADD my own unique identifier then it will create a new record, but I thought the point of the hash plugin was to create the unique identifier. This is a great idea but I think I must be doing something wrong or missing a key point/setting. Any help would be appreciated. Thanks.