Common Venting

Is there such a thing as an inline flapper if you put a Y in the duct to outside so air doesn't go back?

In general, no. A damper such as you describe would be a dangerous obstruction.

Some appliances have powered dampers to prevent backflow. Those appliances also have controls setup to prevent operation if the damper fails to open completely. I don't know if a powered damper can be used on a shared vent.