my zombies named "noobs" attacking each others?

Hi, when function dmg() is run, it checks hit.Parent for humanoid:

local hum = hit.Parent:FindFirstChild('Humanoid')

Your zombies likely have humanoids in them, so they pass the check. You should make another if statement such as:

if not hit.Parent.Name == "noobs" then -- or whatever the zombie name is

to make sure it doesn't run if it hits another zombie. This assumes you want to damage any humanoid. If it's just players, use DeceptiveCaster's solution.

Edit:

local rarm = script.Parent:FindFirstChild('Right Arm') or script.Parent:FindFirstChild('RightUpperArm')
local larm = script.Parent:FindFirstChild('Left Arm') or script.Parent:FindFirstChild('LeftUpperArm')
local head = script.Parent:FindFirstChild("Head")

local debounce = false 

local function dmg(hit) 
    if (hit.Parent ~= nil) and (not debounce) and (not hit.Parent.Name == "noobs") then 
        debounce = true 
        local hum = hit.Parent:FindFirstChild('Humanoid')
        if (hum ~= nil) then
            hum.Health -= 200
        end
        wait(1) 
        debounce = false 
    end
end

rarm.Touched:Connect(dmg) 
larm.Touched:Connect(dmg) 
head.Touched:Connect(dmg)

Players:GetPlayerFromCharacter()

GetPlayerFromCharacter() is a function of the Players service that returns a Player object associated with the given Instance, given that there is a client associated with the Instance. If no Player object can be obtained from the Instance, the function returns nil.

This is extremely useful for zombie AI and you should absolutely use it (I use it myself). You have to counteract the function, such as doing...

local inst = script.Parent
if not game:GetService("Players"):GetPlayerFromCharacter(inst) then
    print("No player")
else
    print("There is a player here")
end

not will essentially check if the function does not return a player; this is equivalent to false, and in this case it refers to not true, which is the same exact thing.