Just a guess,,that's all we can do!,,but I'd first think that recoil is knocking the second bbl sear out of engagement with the hammer.
Maybe the second bbl sear isn't in complete engagement anyway from a problem like Skeettx mentions,,but that's where I'd start looking.
Could be something as simple as dried lube in the hammer sear notch so the sear nose can't get a full bite. Clean the sear areas as skeettx says;
The fact that it doesn't double when dry fired and didn't when using the lighter 3/4oz loads 2 or 3 years prior and had gone unused till now, just points me in that direction.
