You can exclude people from receiving the same email by including this statement in a Smart List:
Was Sent Email IS "X"
Then in the Campaign, say
Member of Smart List NOT IN "Was Sent Email IS X" to exclude anyone who received the email.
NOT Was Sent Email IS "X" will return everyone who never received that email.
btw: Was Sent Email IS NOT "X" will return everyone who received any email BUT "X"
In general, Marketo is oriented around the lead object rather than the email object, so while this is a simple thing in many ESPs, its a feature of Marketo that this would be deduplicated.
Do you have Revenue Cycle Analytics? The Revenue Cycle Explorer module has a type called "Email Analysis" that may help you here.