What do you mean by "duct"? Is is really stainless like 304 (18-8) or 316? Or is it HVAC duct? If the latter, it may be stainless because it is galvanized carbon steel rather than stainless. If it is galvanized, I would recommend not trying to weld it. The only thing worse than the weld quality will be your lungs.
If it is stainless, you can weld it. I patched a mower deck with a scrap piece of 316L. If you use regular weld wire, the weld won't be stainless.
If I were you, I would get 18 ga. carbon steel sheet.