Developer Product Double Buying Bug?

I followed a tutorial on the roblox wiki on how to use developer products as I have never worked with them before... It is working and doing what I want it to do... however... If you buy it multiple times (which you should be able to because you are buying in-game currency) the value is added again each time.

For instance:

1st buy: $1000 (the default value) - Total is now $1000

2nd buy: $2000 (added another $1000 to purchase) - Total is now $3000

3rd buy: $3000 (added another $1000) - Total is now $6000

So instead of ending up with $3000 at the end because you bought it 3 times, you end up with $6000. Does that make sense? Okay heres the script what should I fix to prevent this?

-- setup local variables
local MarketplaceService = Game:GetService("MarketplaceService")
local ds = game:GetService("DataStoreService"):GetDataStore("PurchaseHistory")
local productId = 36573978

-- define function that will be called when purchase finished
MarketplaceService.ProcessReceipt = function(receiptInfo) 

    -- find the player based on the PlayerId in receiptInfo
    for i, player in ipairs(game.Players:GetChildren()) do
        if player.userId == receiptInfo.PlayerId then


            -- check which product was purchased
            if receiptInfo.ProductId == productId then

                -- handle purchase
                player.leaderstats.Simoleons.Value = player.leaderstats.Simoleons.Value + 1000
                print("success")
            end
        end
    end 

    -- record the transaction in a Data Store
    local playerProductKey = "player_" .. receiptInfo.PlayerId .. "_purchase_" .. receiptInfo.ProductId
    ds:IncrementAsync(playerProductKey, 1)  

    -- tell ROBLOX that we have successfully handled the transaction
    return Enum.ProductPurchaseDecision.PurchaseGranted     

end

Edit: Leaderboard script:

print("Sim stuff running!")

function onPlayerEntered(newPlayer)
    wait(.5)
    local stats = Instance.new("IntValue")
    stats.Name = "leaderstats"
    local stats2 = Instance.new("IntValue")
    stats2.Name = "Tycoon"

    local cash = Instance.new("IntValue")

    cash.Name = "Simoleons"
    cash.Value = 10

    cash.Parent = stats
    stats2.Parent = newPlayer   
    stats.Parent = newPlayer
end

game.Players.ChildAdded:connect(onPlayerEntered)