Depends on which dull coat you used.
If's it's an acrylic one, yes, ammonia or any ammonia containing products (Windex) should remove it.
If it's the MM lacquer based stuff, not likely, as it probably etched into the clear parts already. Another good reason for dipping in Future first, as any successive coats is really on the Future, and you have a better chance of saving the canopy. Sanding and polishing and then dipping in future may save the clear parts, if not etched too deeply. You could try to polish it off using old fashion toothpaste also, not the gel type.

Good Luck.