Could be a chicken/egg paradox. The action depends on frame integrity to keep the sear noses in the hammer notches. Let us suppose that a crack opened along the frame, likely through one of the holes that the axles go through, and expanded just enough during firing to allow the sear to slip from the notch... might be difficult to discern from doubling.

Point is that the frame crack may have caused the double discharge. Especially possible if this is the first time the gun has doubled.