I think what is happening is the first blow is setting the cap and the second blow is then firing the cap. The trouble is you can not depend on this to be the case all the time.

First off, on any CVA or Tradition's rifle I replace the nipple with a hot shot nipple. They are stainless steel and tend to throw more fire into the channel.

Something you could try is get some emery cloth and start to buff that upper part of the nipple in the rifle making it smoother and smaller. This will allow the cap to go down further when you first set the cap.

Another thing to do is before you cap the rifle, make sure the nipple is wiped clean with a small patch and some isopropyl alcohol. This will make the nipple as clean as when you first use it. Does it fire the first time you put it on, and then gets worse the more you shoot it?

I would start with the new Hot Shot Nipple. RMC Sports is where I get mine. They work great.

http://www.rmcsports.com/catalog.htm

The last thing it could be is a weak hammer spring but I doubt that as it is firing the cap on the second hit. Try taking some kind of pad and with your thumb push good and hard when you set the cap to try and force it down as far as possible.

Keep us posted. What kind of rifle did you get? And WELCOME TO THE FORUM.. GOOD TO HAVE YOU.