How do i make GUI seperate for each player?

My GUI is broken because when i load up a game w/ my brother I will click Shop to open the shop but the GUI then opens on his screen, and then we cannot close it. Here is the script i use...

dUKey = "PlayerDamageUpgrades"
hUKey = "PlayerHealthUpgrades"
sUKey = "PlayerSpeedUpgrades"
 
game.Players.PlayerAdded:connect(function(player)
    if player:WaitForDataReady() then
	
	    
	    u = Instance.new("IntValue")
	    u.Name = "upgrades"
	    u.Parent = player
	
		    
	    dU = Instance.new("IntValue")
	    dU.Name = "DamageUpgrades"
	    dU.Parent = u
        dU.Value = player:LoadNumber(dUKey)
		    
	    hU = Instance.new("IntValue")
	    hU.Name = "HealthUpgrades"
	    hU.Parent = u
        hU.Value = player:LoadNumber(hUKey)
		    
	    sU = Instance.new("IntValue")
	    sU.Name = "SpeedUpgrades"
	    sU.Parent = u
        sU.Value = player:LoadNumber(sUKey)
    end
end)

game.Players.PlayerRemoving:connect(function(player)
    if player:FindFirstChild("upgrades") then
        player:SaveNumber(dUKey, player.leaderstats.dU.Value)
        player:SaveNumber(hUKey, player.leaderstats.hU.Value)
        player:SaveNumber(sUKey, player.leaderstats.sU.Value) 
    end
end)

name = "Dmg Upgrade 1"
cost = 1 -- the price.
currencytype = "StatPoints" -- this is the value in the leaderboard the weapon is paid with.

game.Players.PlayerAdded:connect(function(nP) -- Connect event to anonymous funtion
	nP.CharacterAdded:connect(function(nC)
		sG = Instance.new("ScreenGui", nP.PlayerGui)
		sG.Name =  "ShopGui"
		fr = Instance.new("Frame", sG)
		fr.Position = UDim2.new(0,0,.5,-15)
		fr.Size = UDim2.new(0,150,0,60)
		fr.Style = "ChatBlue"
		tb = Instance.new("TextButton", fr)
		tb.Size = UDim2.new(1,0,1,0)
		tb.Style = "RobloxButtonDefault"
		tb.TextColor3 = Color3.new(1,1,1)
		tb.Text = "Shop"
	    tb.MouseButton1Click:connect(function()
			tb.Transparency = 1
			cl = Instance.new("TextButton", fr)
			cl.ZIndex = 2
			cl.Size = UDim2.new(1,0,1,0)
     		fr.Style = "ChatRed"
			cl.TextColor3 = Color3.new(1,1,1)
			cl.Text = "Close"
			cl.Style = "RobloxButtonDefault"
			cl.MouseButton1Click:connect(function()
				cl:Remove()
				frdUB:Remove()
				tb.Transparency = 0
				fr.Style = "ChatBlue"
			end)
			frdUB = Instance.new("Frame", sG)
			frdUB.Name = "DMGUpgradeFrame"
			frdUB.Position = UDim2.new(0,150,.5,-60)
			frdUB.Size = UDim2.new(0,150,0,60)
			frdUB.Style = "ChatGreen"
			dUB = Instance.new("TextButton", frdUB)
			dUB.Size = UDim2.new(1,0,.5,0)
			dUB.Style = "RobloxButtonDefault"
			dUB.TextColor3 = Color3.new(1,1,1)
			dUB.Text = "Upgrade Damage by 5"
			cdU = Instance.new("TextLabel", frdUB)
			cdU.Position = UDim2.new(0,0,.5,0)
			cdU.Size = UDim2.new(1,0,.5,0)
			cdU.BackgroundColor3 = Color3.new(1,1,1)
			cdU.Text = "Cost: 1 Skill Point"
			dUB.MouseButton1Click:connect(function()
				dUB:Remove()
				cdU:Remove()
				y = Instance.new("TextButton", frdUB)
				y.Size = UDim2.new(.5,0,.5,0)
				y.Position = UDim2.new(0,0,.5,2)
				y.TextColor3 = Color3.new (0,1,0)
				y.Text = "Yes"
				y.Style = "RobloxButtonDefault"
				n = Instance.new("TextButton", frdUB)
				n.Size = UDim2.new(.5,0,.5,0)
				n.Position = UDim2.new(.5,0,.5,2)
				n.TextColor3 = Color3.new (1,0,0)
				n.Style = "RobloxButtonDefault"
				n.Text = "No"
				rS = Instance.new("TextLabel", frdUB)
				rS.Size = UDim2.new(1,0,.5,0)
				rS.BackgroundColor3 = Color3.new(0,0,0)
				rS.TextColor3 = Color3.new(1,1,1)
				rS.Text = "Are you sure?"
				y.MouseButton1Down:connect(function()
					s = script.Parent
					local l = s:FindFirstChild("leaderstats")
					if l then
						local m = l:FindFirstChild(currencytype)
						if m then
							if m.Value >= cost then
								m.Value = m.Value - cost
								local pS = Instance.new("ScreenGui", s.PlayerGui)
								pS.Name = "Purchase Successful"
								pSF = Instance.new("Frame", pS)
								pSF.Position = UDim2.new(0,0,.3,0)
								pSF.Size = UDim2.new(0,150,0,30)
								pSF.Style = "RobloxRound"
								pSF.BackgroundColor3 = Color3.new(0,1,0)
								pST = Instance.new("TextLabel", pSF)
								pST.Position = UDim2.new(1,0,1,0)
								pST.Size = UDim2.new(1,0,1,0)
								pST.TextColor3 = Color3.new(0,0,1)
								pST.Text = "You just purchased "..name.." for "..cost.." StatPoint!"
							elseif m.Value < cost then
								local pF = Instance.new("ScreenGui", s.PlayerGui)
								pF.Name = "Purchase Failed"
								pFF = Instance.new("Frame", pF)
								pFF.Position = UDim2.new(0,0,.3,0)
								pFF.Size = UDim2.new(0,150,0,30)
								pFF.Style = "RobloxRound"
								pFF.BackgroundColor3 = Color3.new(1,0,0)
								pFT = Instance.new("TextLabel", pFF)
								pFT.Position = UDim2.new(1,0,1,0)
								pFT.Size = UDim2.new(1,0,1,0)
								pFT.TextColor3 = Color3.new(0,0,1)
								pFT.Text = "ERROR: You do not have enough StatPoints for that!"
							end
						end
					end
					y:Remove()
					n:Remove()
					rS:Remove()
					dUB = Instance.new("TextButton", frdUB)
					dUB.Size = UDim2.new(1,0,.5,0)
					dUB.Style = "RobloxButtonDefault"
					dUB.TextColor3 = Color3.new(1,1,1)
					dUB.Text = "Upgrade Damage by 5"
					cdU = Instance.new("TextLabel", frdUB)
					cdU.Position = UDim2.new(0,0,.5,0)
					cdU.Size = UDim2.new(1,0,.5,0)
					cdU.BackgroundColor3 = Color3.new(1,1,1)
					cdU.Text = "Cost: 1 Skill Point"
					wait(2)
					pF:Remove()
				end)			
			end)
		end)
	end)
end)

Thanks for the help but with your fix could you please tell me why it happened, what happened, how to prevent it in the future, and basically anything important that will help me become a better scripter because I REALLY REALLY want to master this. Thanks again mates :)!