301 iRule redirection generating an extra 301 status code?!?

If I had to hazard a guess I'd say that you have two Virtual servers at play here. Based on the iRule posted I would expect https://sitename.com to redirect to https://www.sitename.com, so that I can't explain. I am guessing that you have another virtual server listening for connections to www.sitename.com:80 and redirecting to :443, which accounts for the second. My recommendation would be to add some logging to your iRule and take an httpwatch capture while it's connecting. You can then compare the logs on the bigIP with the httpwatch and see what is actually happening.