Most admissions committees will only consider your highest score, so don't worry about the second test. You should retake the GMAT if you have time and believe that you can do significantly better, and if the average for your target schools is significantly above 660.

What do you mean by significantly above?

When you apply they will see ALL of your scores. If you scored lower on the 2nd attempt then that's not an issue because they're worried about what they will report and that would be the 660. Any improvement at this point would be a smart move - depending on where you apply. If it's a top school then remember that you are competing against the top people in the world! If you think you can do better then why not try?