"Double Tap" Also shortened as DT for slang with the TF2 Cheaters. It is used to shoot a 2 shot burst fire to such Hitscan weapons amongst the TF2 weapon selection such as the Minigun, Shotgun, Soda Popper, Force of Nature, etc. The Force of Nature is the most popular weapon of choice for Double Tap because it can shoot both 2 buckshot projectiles in 1 shot and do 200+ damage long range combined with the additional Crit Cheats if used correctly. They are most popular with Cheat Clients such as Rijin.solutions, Lbox, Fedoraware, Nitro, and so on.
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Here are some videos that show how Cheaters use Double Tap

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Here is a video on someone who is also explaining how Double Tap (DT) Works! Thank you Vadographer for making a detailed video on Double Tapping Cheaters.

You will most likely find out if someone is double tapping from the Sound of the Force of Nature weapon of its quick fast 2 shot firing speed versus the normal semi-fast paced 2 shot Force of Nature shots, and if you hear the Scout in question using a Force of Nature that you suspect using Double Tap, Such as the video above from Vadographer, he shows a visual and audio representation of what a Double Tapping Force of Nature Scout sounds like. People using Double Tap on weapons such as the Minigun is harder to tell because the weapon generally sounds the same from my experience on cheaters using Double Tap on the Minigun in-game or in cheating videos. There is one way you can also see if someone is using Double Tap, is if you die from them! Yes some Scouts will literally kill Soldiers or even Heavies sometimes in 1 shot, which is not normal. So that can easily be proved if you are that Soldier or Heavy.

Unfortunately, there is not much you can do to fight this. Unless the Cheater using Double Tap isn't too experienced and can make slip ups of not being too aware of their surroundings, you are better off looking for another game or trying to convince the enemy team that they are cheating and using a cheat called Double Tap to get them kicked and continue with your Casual Match.