“....allowed the cartridge to get a running start into.....struck the breech face...”

Maybe I’m too thick to truly understand, but the physics involved in firing a double rifle do not lend themselves to a case slamming into the breech face and impacting a high primer.
Quite the opposite, upon recoil the cartridge on the non firing side will slide farther into the chamber to wherever headspace allows and not backwards to the breech face
(Newtons 1st Law: a body at rest...ie: the cartridge; .... and Newtons 3rd Law: equal & opposite reaction and all that.)

.....and...I presume this was a 209 type shotgun primer where the actual cup of the primer is enclosed by the battery cup part?
Unlike a standard large rifle primer it would seem very difficult for a 209 type shotgun primer to “slamfire”....

That your rifle doubled there is no doubt......very weird circumstance.

- Mike