Any idea what would cause it to double?
If the same barrel was fired first both times when it doubled, and that is the sequence it always happens in, I would suspect the tumbler/sear relationship on the second barrel's lock. Oftimes the sear notch has become rounded where the sharp edge should be, and just needs stoning to recut the proper geometry. But, there are many other ills, concerning the geometry, that can be out of whack.
That said, the only way to know for sure is to take it to a good trigger man/smith. I have learned to do most of my trigger work myself, but the learning curve is tough and tedious.
If you don't have a burning desire to learn trigger work you would be much better off leaving it to a pro.
SRH
Addendum - Try removing the second barrel's lock and cleaning it thoroughly, then lightly lubing it. It is entirely possible that crud or debris have gotten into the sear/tumbler and are causing poor engagement.
