Can't delete one specific contact

I found a double contact in the contacts of my iPhone that I can't delete. It's not in iCloud nor in the contacts of my Mac. When I deactivate iCloud for contacts, all contacts disappear except this one.

When I go to "Edit" and press "Delete contact" it jumps back to the list of contacts and the contact is still there. I can change everything in the contact but I can't delete it.

I even reset my iPhone but when I copy the backup back to the phone - yep, here we go again, mysterious 0-contact.

Any idea how to get rid of this account apart from losing the backup and build it up from scratch?

What might help: When I switched from my old 3GS to the new iPhone 5, I suddenly had local and iCloud contacts in the contacts app of my Mac. To get rid of this parallel presentation, I deleted all my contacts in iCloud and removed the iCloud card from all my contacts. After that, I synced it again with iCloud and it looked good - except that there was one double entry now.