The same thing happened on my CVA Mountain Stalker which is a Bobcat. I am sure you are on the right track and that it is a nipple problem. I took Emory paper and just worked a nipple over one night. It helped but as the nipple gets dirty it starts to do the same thing, the first hit sets it the second fires it.

When it gets the end of the shoot day, I open the hammer, swab the bore, dry it, then cap it and actually drop the hammer on the empty rifle to set it. 90% of the time it sets the cap. Then I load it as normal, and I know then it will fire.

But it sure is an aggravation.