Hi Team,
I just received a support case referencing this thread, so I wanted to clarify.
Marketo has a feature called durable unsubscribe, which means that when an email address is set to unsubscribed then it is unsubscribed everywhere it exists. All duplicates with the same email will be unsubscribed if one of them unsubscribes, and all leads created at any point in the future will also be automatically unsubscribed upon creation.
This does not work the same way for Marketing Suspended though, so you would need to set each duplicate Marketing Suspended manually.
Each Marketo campaign will also only send email to a lead with a duplicate one time. If both leads qualify for the campaign, and they are both able to receive email(i.e. not unsubscribed, marketing suspended, blacklisted, etc), the campaign will still only send the email to the first one through. After that any duplicates through the flow step will not be sent the message. If the duplicates are all unsubscribed or otherwise unable to receive email, then the campaign will not send to any of them.
John