Catching Customer update from Marketing Capaign

I would like to update a custom record after a customer's record has been updated after they responded to an email marketing campaign.

What I am thinking is to write a script that deployed on the Customer record which on edit will save the info to the custom record. But how do I limit this update to only when the Customer record is updated from a Marketing Campaign?

Full story (for those with time):
I have created custom entity fields on the Customer which gather custom information from an Online Customer Form used in an email Marketing Campaign.

The problem is that this forms a 1:1 relationship as in if we send the campaign to two different contacts from the same Customer the two contact responses will overwrite each other on the Customer record. Therefore only the last to reponds info will only be saved for future reference.