I wish I could be a lot more precise than I'm actually able to be in this post. Sometimes, when I cast Thor's Fulminaric Bolt, monsters get two turns after my one cast. I am certain I'm not casting twice - the first few times I figured I had accidentally mashed right-click and somehow queued up another one, but I've become a lot more careful about it. It doesn't seem to be related to the number of monsters on screen, since it's happened while clearing a Zoo and also while fighting a Chest of Evil mob one-on-one. It doesn't seem to be floor-related, since it happens periodically. The only guess I have is that it's somehow related to the splash damage, since it doesn't seem to be happening with any other spell. Are there any similar reports? I couldn't search the forum for "twice", since apparently it's too common a word, but "move" and "double" and "cast" and any part of TFB's name didn't return anything similar. I would assume it's operator error somehow. Is there a way I can produce a log of what has happened for the last 5-10 turns the next time it happens to me?
Are you hitting yourself with the splash? It has a stun chance, and if you get stunned it visually looks like the monsters get two turns.