I've tried it with and without the lines separating them. Both add the second code tag for some reason.
I've checked, double-checked, and triple-checked. When I submit the post, there is only one set of code tags.
EDIT: I just tried again on my above post and it worked this time, however I do know for a fact that all of the other 10+ times I tried this, there was only one set. I guess it was some weird temporary glitch.