When saving emails MacroView DMF and MacroView Message will by default ensure only one copy of each email is saved in any one location by uniquely naming the email so that on any additional attempts to save it can identify that it already exists.
The default naming format is a combination of the senders name, the date/time (to the second) the email was sent and the first 20 characters of the emails subject. This ensures that each email is uniquely named because a duplicate would only be possible if a single user could send two different emails with the same subject in the same second.
Refer to Defining the default filename for saved emails for information on how to change the default naming format.
So why are two copies sometimes saved?
When an email is saved as it is sent and then again by the recipient of the email, the two versions emails are actually different. For example the sent copy may contain CC addressees that will not be present on the received copy of the email.
This is also the case in Outlook and is why when you send yourself an email a copy is saved in both your Sent Items folder and your Inbox.
When a sent email is saved using MacroView DMF or MacroView Message, either by an outgoing email filing rule, by using the Save on Send option or by drag an dropping the email from your sent items folder in Outlook, the sent copy of the email is saved.
When the received copy is save because it matches an active incoming email filing rule, or when any of the emails recipients save the email, the received copy is saved.
The default filename used by MacroView DMF and MacroView Message when saving emails ensures that the name of the received copy is the same for all recipients so it is only saved once (by the first user) to and single location in SharePoint. All other recipients attempting to save the email to the same location in SharePoint will still have the Save to SharePoint category and the save location added to the email in Outlook, as if they had save the emaiil, however an additional copy of the email is not save.