The archiving process keeps track of which attachments are archived. If the identical attachment is found to be attached to a different message, the attachment is archived only once, and the second message is archived as a reference to the first. This reference is automatically resolved when the item is retrieved, so there is no impact on the administration or use of the archive.
No configuration is required for this feature. The only significance is that an archive is smaller than if this feature were not implemented. If a large attachment is sent to a large number of mailboxes that are all archived at the same time, the space savings is significant.