How do I stop user from cloning multiple casings?

Use something called a debounce.

Debounces prevent scripts to trigger their function over and over again. Like in a touched script, like this one...

Workspace.Button.Touched:connect(function(hit)
    print("Button pressed") --Print the message
    wait(1)                 --Wait for 1 second
    print("Hi :D")          --Print another message
end)

You'd think this would only print

Button pressed Hi :D

but in reality It would say

Button pressed Button pressed Button pressed Button pressed Button pressed Hi :D Hi :D Hi :D Hi :D Hi :D

In order to stop that we need a debounce. I've fixed the script below.

repeat wait() until script.Parent.Parent.Vehicle.Value~=nil

local debounce = false --DEBOUNCE HERE

db=false
held=false

v=script.Parent.Parent.Vehicle
f=script.Parent
missile=script.Parent.HE
e=script.Parent.Eject

function fireguns()
v1=v.Value.Gun1Bullet:clone()
vel=Instance.new("BodyVelocity")
vel.Parent=v1
v1.Velocity=v1.CFrame.lookVector*500
v1.BodyVelocity.maxForce=Vector3.new(4000, 0, 4000)
vel.velocity=v1.Velocity
v1.CanCollide=true
v1.Transparency=0
v1.Damage.Disabled=false
v1.Parent=workspace
end

function launchmissile()
v.Value.Effect.PointLight.Enabled = true
v.Value.Effect.Smoke.Enabled = true
v.Value.Effect.Fire:Play()
v.Value.Effect.Transparency = .5
v.Value.WER2.Value.C0 = v.Value.WER2.Value.C0 - Vector3.new(0,0, .5)
v1=v.Value.Missile:clone()
v1.CFrame = v.Value.Missile.CFrame * CFrame.new(0, 0, -5)
vel=Instance.new("BodyVelocity")
vel.Parent=v1
v1.Velocity=v1.CFrame.lookVector*500
v1.BodyVelocity.maxForce=Vector3.new(4000, 0, 4000)
vel.velocity=v1.Velocity
v1.CanCollide=true
v1.Transparency= 0
v1.Heat.Disabled=false
v1.Parent=workspace
wait(.1)
v.Value.WER2.Value.C0 = v.Value.WER2.Value.C0 + Vector3.new(0,0, .1)
v.Value.Effect.Transparency = .6
wait(.1)
v.Value.WER2.Value.C0 = v.Value.WER2.Value.C0 + Vector3.new(0,0, .1)
v.Value.Effect.Transparency = .7
wait(.1)
v.Value.WER2.Value.C0 = v.Value.WER2.Value.C0 + Vector3.new(0,0, .1)
v.Value.Effect.Transparency = .8
wait(.1)
v.Value.WER2.Value.C0 = v.Value.WER2.Value.C0 + Vector3.new(0,0, .1)
v.Value.Effect.Transparency = .9
wait(.1)
v.Value.WER2.Value.C0 = v.Value.WER2.Value.C0 + Vector3.new(0,0, .1)
v.Value.Effect.Transparency = 1
v.Value.Effect.PointLight.Enabled = false
v.Value.Effect.Smoke.Enabled = false
wait(.5)
end




function missileclick()
if not debounce then --CHECKS FOR DEBOUNCE
debounce = true --Turns the debounce true to stop it from functioning again.
print("click missile")
if db==true then return end
db=true
missile.Text="Reloading"
e.Disabled = true
launchmissile()
v.Value.Effect.Reload:Play()
wait(3)
missile.Text="Fire HEAT"
v.Value.Effect.Reload:Stop()
e.Disabled = false
db=false
debounce = false --wait 3 seconds to allow it to function again.
end


function gunsdown()
held=true
while held==true do
fireguns()
wait()
end
end

function gunsup()
held=false
print("up")
end


missile.MouseButton1Click:connect(missileclick)
sabot.MouseButton1Click:connect(sabotclick)
guns.MouseButton1Down:connect(gunsdown)
guns.MouseButton1Up:connect(gunsup)