Suppose I buy insurance on the full value of your house from Insurance company A. Then I buy insurance on your house from Insurance Company B. If your house burns down, then I make double the value of your house.
But doing this will put me in jail, because it creates an incentive to burn down your house.
It's a similar deal with CDS's. they created a financial incentive to burn down our economy.
So you buy a CDS on some IBM debt. How do you burn down the house of IBM?