=OBSOLETE=
I have re-done the model from scratch to make it fit in the Spy's hand better.

Be sure...

Be double sure!

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Just a couple ideas that I had in mind when I was creating this:

1) On successful Back-Stab, anyone connected via medi-beam will be stunned.
Stun Duration dependent on the level of Uber-Charge. (100% uber = 6 second stun, 0% Uber = no stun)
Unable to be healed by enemy medics or dispensers while disguised

This works with the name (The double tap) in that it hits two things at once.

2) On a successful backstab of an engineer, all of his buildings will be sapped
Sapper only disables buildings and doesnt do damage
No normal sapper

So, you won't have sapper at all, but back stabbing an engie will disable all his buildings until he (or another engie) knocks the sappers off.

Personally, I prefer the first effect, however, these are just ideas