According to Greener's book(The Gun etc.), the left trigger's sear engagement is made stronger than the front trigger's sear engagement to prevent the left barrel from firing when the front trigger is pulled.

The front (right) trigger is supposed to be the first one fired, according to Greener and thus has a "lighter" trigger pull than the rear trigger, for accuracy reasons.

Stands to reason, that with extra recoil of heavy loads, that if firing the left b. first, it may double, but not of firing the right barrel first.

Just going on memory here, which has not proved to be infallible, lately.