Two emails receiving the same Message ID
There is a history of Message-ID issues with Outlook.
There were reports that Outlook 2003 dispatched e-mails without the Message-ID headers to non-Microsoft MTAs.
There is a feature called Outlook forms that lets one create a template and send messages from a template. All messages have the same message id.
Outlook does not store message-ids of the sent messages.
The problems might be with third-party systems that remove duplicate e-mails from mailboxes, list servers that would not resend a message that they have already seen, systems that trace conversation threads, since there will be multiple messages in a mailbox to link to an 'in-reply-to' reference.
Omitting a message-id altogether is a safer option, since as of the RFC 2822 the existence of a message id is an optional (SHOULD have) requirement, but its uniqueness if it is supplied is necessary (MUST be).