This question is difficult as only the miners can answer it 100% correctly, but let me try:
You need to differentiate between:
- successfull double spendings
Double-Spending is not really possible because of the consensus. The miners need to check wheather the bitcoins are still unspent.
But: The scenario of a chain-fork (because of a 51% attack or a bug) could cause a temporary double-spend-problem. This guy tells about a bug that caused a chain-fork of 26 blocks and multiple double spends (but that seems to be happend only once).
- and only trying to double-spend
It's not illigal to try it (so far, as I know). I tried it because I wanted to know how it works. I think, a lot of people experimented to send the same bitcoin to 2 or more different addresses at the same time but of course it did not work.