Brian,
I have had several rifles that have had a problem with the cap needing two hammer falls befor it would go off. Most of these have been either one of, or a combination of 1) weaker than usual mainspring, and 2) a nipple that has a taper thick enough that the cap is starting onto the tip of the nipple but is not bottoming out due to the skirt of the cap. What could be happening is that the cap is not bottoming out on the nipple and the first hammer fall is seating the cap fully onto the nipple and the second hammer fall is igniting it. If this is happing only once in ten shots, you will find that as the shooting session progresses, it will become more frequent due to fouling on the outside of the nipple making the cap harder to fully seat. A cheap fix for this would be to 1) chuck the nipple in a drill and spin some of the taper out of the nipple using a file or sand paper, making it easier for the cap to seat fully down. This should also put a smoother finish on the outside of the nipple, also making it easier for the cap to seat. 2) Switch to a different brand of caps. I've noticed that some caps will have a tighter skirt on them than others, even though they are #11's. In my experience the old Remington #11 caps, now hard to find in this part of the country, had a partial split in the bottom of the skirt that made them seat easier onto the nipple, CCI #11 caps dont have that split and will not seat as far onto the nipple, and the CCI #11 Magnums seem to have a little tighter skirt than the regular #11's. If the main spring is a little on the weak side, it could be replaced with a stronger one that will swedge the cap onto the nipple and ignite it consistantly in one hammer fall.
AndyHass has given you some good advice to check the area under the nipple and in the snail/drum. The CVA rifles that I have had apart, not Mountain Rifles but St. Louis Hawkens, have had a long flash channel so there is ample opertunity for oil or other cleaning products to settle there, especially if the rifle is stored butt down.
These are just some of my thoughts on your problem. Take them for what they are worth to you.