Lead belonging to multiple campaigns: effect on stats?

Suppose, I have 2 campaigns; “Facebook Campaign-1” and “Direct Mailer Campaign-1”.

Both campaigns drive potentials leads to a person form to create a lead in a CRM. What happens if the lead responds to the form twice, and I then merge the duplicate leads?

Would that person belong to both campaigns?

Then, if they belong to both campaigns, what happens if the deal gets Closed-Won? How does that affect ROI tracking? Do both campaigns get “credited” for closing the deal?

All data under all subpanels are properly merged on the choosen primary record, so you will have the unified campaign log records for both Leads under the very same one.
But you need to take care of the field Campaign (campaign_name), you have to enable merge duplicates on such field so you can save the previously fullfilled one.