Hey Sharon,
One way would be to create a cusotm Mkto field that says that they are a part of the campaign. Then, add that field as a hidden field on your 2.0 form. Then, in the follow up page settings, set it to "If Member of SFDC Campaign", follow up page is "Already received it" page.
Another way would be to use snippets and segmentation. You could created a simple segmentation of those who are a member of the SFDC campaign. Then, create a snippet that can be used for new Leads and those who are part of the segment. The new Leads would see "thank you for your submission! We will send you an email shortly". Then, in the Flow steps, have them wait for a couple minutes, then send email if they are not a member of the SFDC Campaign. If they are, they would see "Sorry, our records show that you have downloaded this already" and then have them excluded from receiving the email.
Hopefully that makes sense!