TMair, the Zabala you have that had a sear that was only partly engaging due to the sear tail being in contact with the trigger blade had probably been that way ever since it was built. But it took years of use and wear for it to finally reach a point where the jar from firing the first barrel caused it to slip out of engagement. The shrinking of wood that skeetx mentioned, or improperly tightening screws can cause these types of issues too.
That certainly takes some of the craziness out of it, and makes sense, I guess I just assumed it had been correct from the factory.
At any rate I was just throwing a possibility.
Edited to add: the only other gun that I have had double on me was a Felix Sarasqueta with a single trigger, it was just dirty and a good cleaning took care of it.
TM
