I'm glad this question was asked. Because several years ago, I searched for some info on that exact topic. And I struck out. The group I shoot and throw with is very relaxed when it comes to knife and hawk rules. Basically because we are trying to encourage others to join us. Sometimes strict rules discourages newcomers. The knife I throw has one edge. I've only thrown at one vous years ago. Was no problem there. I'm really interested in hearing what anybody has to add.
The only thing I read one time was, the knife had to have an edge the whole length of the blade. But it didn't say anything about using a double edged blade being wrong.