You see that a lead has filled out the Unsubscribe form multiple times but it still receiving emails.
This situation can occur when the same person has two different emails addresses in the Marketo database that both point to the same email inbox.
For example, you send firstname.lastname@example.org an email, which is sent by the receiving server to the inbox for email@example.com. Because Marketo uses email as a unique identifier, it considers firstname.lastname@example.org and email@example.com to be two different leads, even though they both go to the same email inbox.
The lead may receive the email sent to firstname.lastname@example.org but fill out the unsubscribe form with email@example.com. This unsubscribes the firstname.lastname@example.org lead, but emails will continue to be sent to email@example.com.
Inspecting the activity logs for both leads would make it very clear. For firstname.lastname@example.org you will see Email Sent, Delivered, Opened, Clicked, and then for email@example.com you would see Fills Out Form, and Change Data Value Unsubscribed = True.
To resolve this, manually unsubscribe the lead which is still being sent the email, so the person on the receiving end of both email addresses stops receiving Marketo emails.