"Double rage"

"Double rage" units can be built at your biology lab once it reaches level 9. The "double rage" is pretty much exactly what you would expect -- two berserk units for the price of one. Note that when you deploy a double rage, the units that you get are actually berserk units, so they will have the same upgrade level as your regular berserks. See the berserk page for more details about the training and skins available. Possibly because of this special behavior, you can't take them on expeditions.