Stan: By 'over pressure' I mean a shell loaded 'over pressure', i.e. a loading error.

Since the barrel isn't blown where the pressure is normally the highest, rather the 'bulge' shows just downstream of the forcing cone, no 'double charge' would be necessary to achieve the observed results.

But, as you apparently agree, the direct forensic evidence of a double charge is gone. We can only speculate on that, but we can see the bulge.