Folded Over Matza

I have heard that it is a problem if part of a matza is folded over (matza kefula). How much of a problem is this, and what do I do about it?

Practical Aspects of Matzoh Baking

Key Accessibility for Non-Jew Who Buys Chametz

May I Enter the Room that I Sold to the Non-Jew?

The Mayim Shelanu Saga