I suspect that Steven is testing this differently...
Diederik is pushing the 5 separate lead records with the identical email through a trigger campaign individually - so each record will be sent email (assuming not unsubscribed) and there will be 5 copies of the email sent. If it is a batch campaign that is sending out the email, then Marketo's send email flow step will see that the same email address is referenced multiple times and will only send the email once. That's a key difference between a triggered campaign and a batch campaign. So, if you can use batch campaigns and send all the emails at once, you can avoid receiving duplicate emails. Otherwise, there is no way around this problem - each of these records stands alone and will be looked at separately by the Marketo campaign limits (ie run once).
Unsubscribed is handled a bit differently. When an email address is Unsubscribed, Marketo has a system function that looks for all records with that email address and sets Unsubscribed is true for all of them. That way you can be sure if you have duplicates that you are in compliance with CAN-SPAM and other laws.
This is one reason that duplicates and Marketo really do not work well together.... you really do want to clean you database and de-dup the records!