Destruction Banshee’s empowered fireball says that it has a 75% chance to repeat casts of fireball. It can also repeat other spells as well. In the arena, I had a destruction banshee on my team cast cannibalize 8 consecutive times, for example.
There is a similar bug with Valentine Banshee on Windows 10, so this might actually affect all the Banshee traits.
Thanks, fixed in the next patch!