was a problem with the Java script on my PC (no idea what that means, I'm not a techie).
If what you say is true..
It means Ryanair don't have the proper checks and balances in place to prevent their javascript (as it came from them to get to your PC) causing you to be double charged. Their fault, it's easy to blame a computer because non-techies accept this, but the blame rests with their programmers and their internal systems.
I understand that when you dispute a charge, this gets logged against your account, and this information can be accessed by the merchant, and if you do it too many times some merchants will not accept your card. So I've heard...