Eww!

Imagine you've finally found the love of your life. You have so much in common including taste in music, phobias, career choices. Heck, you even both named your dog Toy! You often find yourselves finishing one another's sentences and it's like you can FEEL when something's happening to your soul mate.

Ah, love...

The blessed day of your wedding comes and passes. Then, you get an odd query from the government. The results of your blood test came back with a bit of unusual information.

Your DNA and that of your soul mate matches. Your soul mate is your biological twin.

...

This is exactly what happened to a British set of twins who were separated at birth and happened to meet and inevitably fall in love later in life. Their marriage has since been annulled, but the psychological damage has likely maimed the two for life.