Not with you on this one. You don't double because you have a sure set, and you do not need to believe that 6nt is inferior to 6♦ to double. You double to direct a lead. If 6nt and 6♦ are equal contracts, then you have made the valuable lead-director "free of charge". Even when you suspect that 6nt might be better than 6♦, it isn't always they run. I suspect in practice it's somewhere in between what you and I said. When lefty has bid NT twice (thrice?) already naturally, I think you should expect someone to run to NT more often than not, so you have to be more careful about lightner doubles. I definitely like a style where I can double if I have a void so I can get a ruff, and we'll worry about the second trick later. But you still have to keep the rest of the auction in mind when you do it. An auction like this one, if I were the partner of the doubler and they ran to 6NT, I would certainly expect us to beat this some substantial amount of the time.