Code redeemable after player rejoins even though they redeemed it already?

Problem

Yes, we need to use DataStoreService.

Solution

We don't need to create the BoolValue Instance, we can simply store a bool value in DataStores.

When using Data Stores you always need to use a pcall AKA protected call because Roblox DataStores can fail sometimes.

Fix

local DataStoreService = game:GetService("DataStoreService")


local DataStore_CodeName = DataStoreService:GetDataStore("RedeemCode", "CodeName")
local DataStore_CodeName2 = DataStoreService:GetDataStore("RedeemCode", "CodeName2")

local CodeRedeemed = game.ReplicatedStorage.CodeEvent


local function OnCodeRedeemed(Client, Code)
    local Success, Result


	if Code == "CodeName" then
		
		Success, Result = pcall(function()
			return DataStore_CodeName:GetAsync(Client.UserId)
		end)
		
		
		if Success and not Result then
			Client.leaderstats.Points.Value += 100
			
			Success, Result = pcall(function()
				DataStore_CodeName2:SetAsync(Client.UserId, true)
			end)
			
			if not Success then
				warn(Result)
			end	
		end
	end
	
	
	if Code == "CodeName2" then
		
		Success, Result = pcall(function()
			return DataStore_CodeName2:GetAsync(Client.UserId)
		end)


		if Success and not Result then
			Client.leaderstats.Points.Value += 250
			
			Success, Result = pcall(function()
				DataStore_CodeName2:SetAsync(Client.UserId, true)
			end)
			
			
			if not Success then
				warn(Result)
			end
		end
	end
end


CodeRedeemed.OnServerEvent:Connect(OnCodeRedeemed)

EXPLOITING PROBLEM

I've found a way to easily exploit your game.

I see that you passed a Reward argument through the RemoteEvent. So I can exploit it with an executor and do this

game.ReplicatedStorage.CodeEvent:FireServer(1000000, "CodeName")

And there we go, I just gave myself 1M points. So how do we fix this? Just don't pass stuff like that to the server and do the rewarding on the server.

You can understand DataStores here. It explains everything about DataStoreService and how you can use it. BTW, you don't need to read the whole article all at once. You can read bit by bit otherwise, you'll get overwhelmed. If there's any problems with the code, you can reply to me in the comments on my answer. You should play around with DataStores too, so you can understand and feel comfortable using them.