Best "error" I've ever seen today.

We have a very large database with millions of people, some of those records are duplicates. So I had a long project to write something that would automatically merge duplicate records while allowing employees to review them. Today we had a duplicate show up in the list which should not have.

Same name (apart from one letter), address, employee ID (off by 1 number), same manager, title, phone number, birthdate. But we figured out it wasn't the same person and therefore wasn't a good match.

Turns out they are twin sisters who live together and we're hired by the same manager for the same position at the same time. What are the odds...