I’m assuming that it was an air handler and “A” coil / no furnace unit. There’s a couple of things you can do.
1. All of the attic air ducts must be well insulated, otherwise, the warm air that penetrates registers inside your house, travels into the cooled air ducts and condensation develops on their interior and exterior walls. I’m not sure what kind of a system / type of air ducts you have – there are different types of insulation you can use on square and round ones.
2. If it is AC only system / no heat, seal all of the registers for the cold season – inlets and outlets. Depending on installation, there might be dampers right behind the registers, or if there aren’t any, and there are no louvers you can close, you can get magnetic sheets in your hardware store that stick to metal registers, or use any other material that would prevent warm, conditioned house air from migrating into the air ducts.
3. If this is an AC / Heating unit, well insulated all air ducts and plenum (section containing the “A” coil) should be enough.
Let me know if there is anything else I could help you with