How to make 4 digit code have no repeating numbers?

Trying to get a random 4-digit code for a locked door in my game but I want the password to have no repeating numbers. How would I do this?

Code:

local startCode = game.ReplicatedStorage.startCode

local numb1 = math.random(0, 9)
local numb2 = math.random(0, 9)
local numb3 = math.random(0, 9)
local numb4 = math.random(0, 9)

startCode.Value = (tostring(numb1) .. tostring(numb2) .. tostring(numb3) .. tostring(numb4))
print(startCode.Value)

One way to do this is to make an array of digits from 0 to 9

Digits = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}

We take a random number from Digits, and then remove said number since so that it cannot appear again

RandoNum = math.random(1, #Digits)
Chosen = Digits[RandoNum]
table.remove(Digits, RandoNum)

Then we can just take said number and add it to startCode.Value

If you wanted your code to be in a loop, it would look something like this:

Digits = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}
for i = 1, 4 do
	RandoNum = math.random(1, #Digits)
	Chosen = Digits[RandoNum]
	table.remove(Digits, RandoNum)
	startCode.Value = startCode.Value..tostring(Chosen)
end

Start by making a array/table for every number in the number combo.

local code = {"N","N","N","N"}

Now, let's make a for loop for every part of the table. If there's no value set, ("N") we'll change it to a random number. Next let's make sure no other number in the array/table is that number.

local function check()
 for i, v in pairs(code) do
	if v == "N" then
			return true
		end
end
end
while check() do
	for i, v in pairs(code) do
		if v == "N" then
			local suggestednum = math.random(0,9)
			print(suggestednum)
			local isnotothernumber = true
			for i, v in pairs(code) do
				if v == suggestednum then
					isnotothernumber = false
				end
			end
			if isnotothernumber then
				code[i] = suggestednum
			end
		end
	end
-- if you want it to be a string
 combostr = ""
 for i, v in pairs(code) do
	combostr = combostr..v
 end
end 

So there you are! A randomly generated number sequence that is 4 digits long and 2 digits will never be the same.