How do i use 2 of the same letters in Enum.KeyCode?

This is actually not that difficult to accomplish.

The primary function to use is tick(), which represents the time passed since epoch (June 1, 1970), in seconds. Why is this important? Well, the function is used to time certain things, such as a double jump or double click.

First create a variable that presents initial time:

local InitialTick = tick()

Then use the InputBegan event and check for a desired key:

local uis = game:GetService("UserInputService")
uis.InputBegan:Connect(function(input, gpe)
    if input.KeyCode == Enum.KeyCode.W and not gpe then

Set another variable for the current tick:

local CurrentTick = tick()

Now we check to see if the input received was indeed a double tap, press or click. This is accomplished by subtracting the InitialTick from the CurrentTick. If the time between key presses is either less than or exactly 1 second, then count it as a double tap, press or click.

Here's what that would look like:

local TimePassed = CurrentTick - InitialTick
if TimePassed <= 1 then
    -- Enable sprinting
    game.Players.LocalPlayer.Character.Humanoid.WalkSpeed = 50 -- Any desired number
end

Followed by the other ends:

    end
end)

If you also want the double press to be able to disable sprinting, you can use a boolean value and implement it into your code.

Example of doing this along with complete example script:

local uis = game:GetService("UserInputService")
local IsSprinting = false
local InitialTick = tick()
uis.InputBegan:Connect(function(input, gpe)
    if input.KeyCode == Enum.KeyCode.W and not gpe then
        local CurrentTick = tick()
        local TimePassed = CurrentTick - InitialTick
        if TimePassed <= 1 and not IsSprinting then
            -- Enable sprinting
            IsSprinting = true
            game.Players.LocalPlayer.Character.Humanoid.WalkSpeed = 50 -- Any desired number
        elseif TimePassed <= 1 and IsSprinting then
            -- Disable sprinting
            IsSprinting = false
            game.Players.LocalPlayer.Character.Humanoid.WalkSpeed = 16 -- The original WalkSpeed of the Humanoid
        end
    end
end)