Is a window that condenses necessarily defective?

No. A sound window can condense on a very cold morning if indoor air is humid (dryer, showers, aquarium, low ventilation). Newer, tighter windows can even make indoor fog more visible, for lack of air exchange with outdoors.

When to suspect a defect

Fog between the panes, rainwater infiltration, or extreme condensation on one window while others stay dry under the same conditions.