There is always a redo. I had to have two, seven months apart. My EP was the top trainee for the Canadian Cardiovascular Society for 2002-2003. He frankly warned me that even he has a 25% failure rate for first ablations. His success rate on second attempts was a bit higher than 80%. I won with the odds, but only on the second go.
I know people who have had literally scores, yes 'scores' of cardioversions, mostly because none of them took for long. They have had three, five, six, ablations until they found the right EP. In this one case, six ablations, is was Dr. Andrea Natale at the Texas Cardiac Arrhythmia Institute in Austin, and his patient was an EMT who also knows his 'stuff'.
Your attending cannot have a person handle your treatment unless they ask your permission, or except during an outright emergency when there's nothing left but to have the trainee/assistant step in to do something critical. So, I doubt there is anything poorly or improperly done that could be attributed to a trainee...but I guess anything is possible, or at least plausible. I would scream if I learned of it and had not been advised or asked beforehand.