Non-insulated duct will sweat in all but the driest conditions, the better the A/C works (colder the air) the more condensation on the cold duct.
Insulation is the key, if not insulated then replace with some that is insulated. If it is insulated duct then dry it out and add another layer.
Plastic and wood supply boxes are better then metal as cold metal will condense the humidity in the air and ruin contacted surfaces.
Easy fix, get er done.