Contracts on properties already represented by a realtor

I am curious if anyone has worked with properties that already have a realtor contract on them. In my area, there are several properties that have real estate agent signs in front of them. Having signed on of these contracts myself to sell a house, I am wondering if anyone has ever tried to write a concurrently running contract to assign the deal. The clause I'm referring to is one that prevents the owner from selling the house themselves or requires them to refer buyers to the realtor.

I was thinking of a contract to the effect of allowing you time to find a buyer, but if the real estate agent finds a buyer before you do, your contract is null and void and the previous contract is applied.

Cheers,
Phill