Occasionally heavy condensation on the outside of a double-paned window can be a sign that air is leaking from the inside of your house around the window. Small amounts of condensation are normal as the temperature is different on the outside than the inside. If you are concerned about the amount of condensation have a window installer or your contractor come out and check to see if the window was improperly installed.
Leaking windows can be caused by several problems, but usually a dual-pane window that leaks has a manufacturing defect such as an incorrect seal. Contact the manufacturer of the windows if the leak starts within the warranty time, or check around the window for any signs of damage to the seal.