How can I fix this dev product?

Hiya all. So, what happens when somebody buys this dev product, it awards +10 speed. And when you die, you lose it. The problem is say you have 56 walkspeed because you bought it 4 times, then you die. Say you buy another +10 after you die. It awards you all the old walkspeed as well and the new +10, then giving you 66 Walkspeed (just an example). How can I make it so you can't get all of that back after death? So say if you bought another +10 after death you only have 26 WalkSpeed? Thanks!

This script is in the Workspace, in case anyone needs to know what.

local developerProduct = 20338463 -- Id of the developer product
local speedInc = 10 --Change this to the amount of Walkspeed you'd like to give. 

Game:GetService("MarketplaceService").ProcessReceipt = function(receipt)
    if receipt.ProductId == developerProduct then -- the speed dev product
        for i,v in pairs(game.Players:GetPlayers()) do -- loop through the players
            if v.userId == receipt.PlayerId then -- if they're the player that bought it
                if v.Character and v.Character:FindFirstChild("Humanoid") then
                    v.Character.Humanoid.WalkSpeed = v.Character.Humanoid.WalkSpeed + 10 -- add speedinc to their current walkspeed
                end
            end
        end
    end
end