==========================Original Stats===========================

~ -60 health on consumption
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This sounds very balanced, as since the Heavy has 300 health, the amount of health lost isn't a greatly high amount, but just right for a man of his status.

~ 3 seconds of rage-induced crits
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Most likely not enough time to deal any damage worth the health loss and time it takes to eat the fruitcake. It's like The Killing Gloves of Boxing, instead of taking out your Minigun, winding up the barrels, and then having like a second or two to shoot, you could take out your Shotgun to deal some damage fast. However, The Festive Fruitcake is used in the secondary slot, just like the Shotgun, so that's not an option.

~ Teammates pretend to be thankful on pickup
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Obviously a joke, but there could be a gimmick with The Festive Fruitcake's Alt-Fire.

=========================Reworked Stats==========================

~ -60 health on consumption
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See above.

~ 7 seconds of rage-induced crits
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More time for crits may look and sound overpowered in text, but in actual gameplay, The Killing Gloves of Boxing don't even have much time to get someone even with the 5 seconds after a kill. Having around 5-7 seconds sounds much more fair than just having 3.

~ Alt-Fire to throw a heavy fruitcake at foes
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Throw a fruitcake at your enemies to dish out some high damage and knockback as well as ruining their Holiday Spirit.

~ 20% longer cooldown time
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So fruitcakes aren't spammed 24/7, that would ruin EVERYONE'S Holiday Spirit!