After I attack with cluster missiles the first time ,I know my opponent rolls defense dice. When I attack the second time, does my opponent roll defense dice again?

Yes.

Thx

This has already been covered, but here's a fishing lesson.

Take a look at the timing chart in the FAQ, with special attention to the references to "Perform This Attack Twice" attacks. You'll see which things happen twice (like rolling defense dice) and which only happen once (like declaring a target).