It says add your Sneak Attack damage in addition to the damage caused by your crit.
If he happened to be flat-footed, sucks to be them. If a normal hit was a sneak attack, add the dice, if it's a confirmed crit, add the dice for that ability too.
If a Skullclan Hunter used his sneak attack undead ability, and used Razing Strike, he would also get double sneak attack dice.
The Sneak Attack ability doesn't say 'only once per hit'. Though normally it may be a given, there are at least 2 places that you can double up on it.
It does say extra dice aren't multiplied on a crit, but these aren't, they are added.