If you find condensation on your ductwork, you need to reduce the moisture in the area.
If your ducts are in the attic, you need to make sure the attic is well vented with the installation of soffit and roof vents.
If the ducts are in a basement or crawl space, adding a dehumidifier will help get the moisture out.
Then proper insulation and sealing your ducts will help stop the condensation and also save you money.
Watch this video to see how to properly seal your ductwork.