Doubled pawns are not too bad if you're only trying to defend, so if Topalov had decided to take that knight on e6, leaving Kramnik with pawns on e6, g7 and g6, I don't think he would have been able to exploit them.
The real weakness of doubled pawns in an ending is when you have an extra pawn on one side, but it is doubled. This often means that you can't break through on that side.