Do not harvest files when "replace original file" is checked and file has already been processed

I have the following Media Mover setup:

-Harvest from CCK field.
-No process.
-Store to Amazon S3.
-Complete: save file to original cck field, "replace original file", "delete source file".

This all works great the first time through. However, when cron runs a second time, MM tries to re-process the files that have already been processed (tries to harvest the files from Amazon S3). MM does not succeed, but posts an error like this to log:

Harvested file is not readable, check permissions: http://s3.amazonaws.com/pixeled-memories/Busan Beach 1 - ribbon1_1.zip

Obviously, I don't want these files to be re-harvested at all once they've been moved to Amazon. Any ideas how to go about fixing this?