Thanks for the tip --- Luckily the wife will be going on a half-marathon run tomorrow so I'll have a good chunk of the day to tinker with my rifles. I'll try making some adjustments to the magazine and test with snap caps. Ill try and post a video or photo later on too if it's still jamming up to show you the double feed I'm gettingIt is definitely the mag. I have a bunch of all old metal mags that I picked up over the years, about half would work in my 760 and half wouldn't. One day for a project I went through and watch the ones that didn't work as I brought the action forward. I could see a variety of problems - sometimes it was the R side round that would get stuck, sometimes the left, sometimes two would come up (like you have). I managed to fix all of them with pliers and some gentle nudging of different parts of the metal fins on top. Those steer the rounds into place.
With two popping up in a 4+1 scenario, I'd guess the rear of the fin on the side of the first round (left I think) needs to be rolled in a bit, so the bolt only grabs one round.