I have zero experience with the Nikko (Winchester 101?) but plenty of experience chasing my tail trying to diagnose the cause of my old Valmet's doubling. It turned out that it only doubled when firing the bottom barrel first. The sound it made was kind of a weird echo-like rapport. It was caused by the ever slight delay between the bottom barrel going first and then almost immediately the top. The cause (no surprise to many here) was a weak firing pin return spring on the top barrel. Inertia caused the unfired barrel's pin to want to stay put as the recoil forced the rest of the gun (and the chambered shell) to move backwards. The weak return spring was not stiff enough to overcome the firing pin's inertia. The result was a shell that slammed into the firing pin instead of the other way around.