How do I stop the walkspeed of the character not decreasing after double tapping "W" ?

You need to set click back to false. Otherwise, it will stay true forever, confusing your script. I redid the code so that there are two variables, making it easier for the script. New Code:

isPressing = false
click = false
locplay = game.Players.LocalPlayer
human = locplay.Character.Humanoid
char = locplay.Character
local Anim = Instance.new("Animation")
Anim.AnimationId = "rbxassetid://1453429139"
game:GetService("UserInputService").InputBegan:connect(function(input1)
    if click then
	isPressing = true
    end
    if input1.KeyCode == Enum.KeyCode.W then
        if isPressing then
            print("W was double tapped.")
            char.Humanoid.WalkSpeed = 20
            local playAnim = char.Humanoid:LoadAnimation(Anim)
            playAnim:Play()
            return
        end
        click = true
        wait(0.4)
        click = false
    end
end)

game:GetService("UserInputService").InputEnded:connect(function(input1)
    if input1.KeyCode == Enum.KeyCode.W then
	  if isPressing then
	  	isPressing = false
	  	char.Humanoid.WalkSpeed = 16
	  end
    end
end)