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I’d add that it’s also likely be be against current building standards/regulations.
Shared duct or exhaust systems have to be designed so that the exhaust from one room cannot vent into another room.
This usually involves having an inline exhaust fan in the final section of a shared duct after all the inputs from various rooms/lots have joined it. Or an exhaust fan on the roof itself. This ensures a negative air pressure at all room vents.
I’m not sure how your duct system was permitted without one but it might be due to the age of the building. Also without any fans, it might have replied on a chimney effect from the eventual roof outlet.