After rescuing an email from the greylist, it is delivered to my inbox. However, as soon as it enters the greylist, a reject response was sent to the originating server. Assuming that it is a valid email (and it should be, that's why you rescued it), the server will try again within 15 mins. When the second (and duplicate) email arrives, as it was rescued, it arrives directly to your inbox. Mailwasher should discard this second email as you already have a copy.
