[BoberJones]

 Reproduce:
1.) Have Mercs
2.) Turn off Total Control
3.) Get into battle
4.) Drag your 'attack' from the menu to the hot bar
5.) Hold the hot bar key with attack while your Merc is in total control

Example: GIF
This can make snakes shoot two snake heads, and also make some mobs attack 2-4 times, 2 if they moved, 4 if they didn't

(posting for another because they don't have a forum post)

[adamkad1]

As far as i've observed, what happens here is that the game allows you to force an attack while an animation is playing out. Though I think you need total off for it? And you can, pretty much, still controll the enemy using ability hotkeys while the ai is acting. Heres a better tutorial 
1-Hotkey attack or an ability merc has
2-turn off total control
3-quickly press hotkey and z during an animation (Basic attack , Slithering earth, the likes)
4-Merc acts twice for the cost of one.

(Also you forgot to include version name in the topic, thats kinda important)

[Neus]

The private bug report I received said that Total Control had to be on, which is why I had trouble replicating this. However, it should be corrected in 2.61c.