GDPR compliance tip: Got duplicated contacts in your lists? What happens if they unsubscribe?

If you have duplicates in your mailing lists or CRM database, what happens when you tag one as unsubscribed for GDPR purposes?

 
What will happen to you if that contact gets emailed again?
 
How you deal with this, I suppose, all depends on how well do you want to sleep at night when the GDPR comes into force?
 
If you don’t have all your marketing, mailing and contact data in one list, and tidied up with duplicate contacts combined, you cannot safely ‘unsubscribe’ contacts when they request this - as you’ll tag the contact, but then another version of the contact may get mailed.