According to the rules on page 71 of the Players Guide any double roll (00, 11, 22, 33, 44, etc) is a critical hit. Further, critical hits in the Hit Locations of 0, 1, 2, or 3 are instant kills (which can be avoided by the victim spending a Luck Point, pages 90 and 91 of said Players Guide).
Rolls of 01, 02, 03, and 04 are Lucky Breaks where something beneficial happens. Conversely, any roll of 95, 96, 97, and 98 are Bad Breaks. Rolls of 00 and 99 are the ultimate of each respected extreme (page 8 of the Players Guide).
Here's where these rules are in paradox- in the rules as written the combination of doubles and Hit Locations result in the left leg- Hit Location 9, impossible to score a Critical Hit as it would require a roll of 99 to hit the left leg, but a 99 is an extreme Bad Break.
The work around for this fluke is to make one additional roll. One roll to determine if you hit, this would be the roll that would be susceptible to the Lucky Breaks or Bad Breaks. And then a second roll to see where and how much damage dealt, this roll would be exempt from the Lucky Breaks or Bad Breaks.
Here's a link to the Yahoo Group discussion about this...