External routes in OSPF generate LSA 5, a default behaviour of ospf. What will happen if the same route is being advertised by two asbr. How the far end router selects which route is the best. Remote router will be receiving the route with metric of 20 so may be it will load balance the traffic. This is the CCNA perception. But actually router checks forwarding address. It calculates the cost upto the forwarding address and compare the shortest as best path.
I am writing a full article on to that and definately will post with in few days.