How do I fix my Inventory putting items in wrong slots?

I'm working on my Inventory system for my Roblox survival game and I can't figure out a way to fix an issue I've been having that you can see in this GIF I've made: https://imgur.com/BwLYrmG

I believe I've found what's been causing this bug but I can't find a solution to it. I believe the cause is that when the script runs the function updateDisplay() every second, it checks through all the hotbar and inventory slots and if one of them is empty, it puts the item there. However, if there is only one item in the player's Backpack, the script says the same tool is in slot1 and also puts it in slot2. I'm wondering how I can implement the check for this.

the script is down below:

local StarterGui = game:GetService("StarterGui")
local Players = game:GetService("Players")

StarterGui:SetCoreGuiEnabled(Enum.CoreGuiType.Backpack, false)

local player = Players.LocalPlayer
local backpack = player.Backpack
local char = player.Character

local GUI = script.Parent.Parent.Inventory
local hotbar = GUI.Hotbar
local inventory = GUI.Inventory
local items = inventory.Items

local slot1 = inventory.Items.InventorySlot1
local slot2 = inventory.Items.InventorySlot2
local slot3 = hotbar.Frame.HotbarSlot1
local slot4 = hotbar.Frame.HotbarSlot2

local invSlotData = {
    [slot1] = nil,
    [slot2] = nil,
    [slot3] = nil,
    [slot4] = nil

}


local function scanItems()

    local items = {}

    --checks backpack and character for items and adds them to the Items table
    for i, tool in pairs(backpack:GetChildren()) do
        if tool.ClassName == "Tool" then
            table.insert(items, tool)
        end
    end

    for i, tool in pairs(char:GetChildren()) do
        if tool.ClassName == "Tool" then
            table.insert(items, tool)
        end
    end


    local invItems = {}

    --[[looks through Items table, and if a tool has the same name as an index in invItems table, add to the index's count and insert
    the tool's name into the index's Items table. else, create a new index with the tool's name and set all it's tables to default  --]]
    for i, tool in pairs(items) do
        if invItems[tool.Name] then
            invItems[tool.Name].Count += 1
            table.insert(invItems[tool.Name].Items, tool)
        else
            invItems[tool.Name] = {
                Count = 1,
                Items = {tool}
            }
        end
    end

    return invItems
end

local function resetItems()
    for i, button in pairs(items:GetChildren()) do
        if button.ClassName == "ImageButton" then
            button.Image = ""
            button.CountDisplay.Text = ""
            button.BackgroundColor3 = Color3.fromRGB(255, 255, 255)
        end
    end

    for i, button in pairs(hotbar.Frame:GetChildren()) do
        if button.ClassName == "ImageButton" then
            button.Image = ""
            button.CountDisplay.Text = ""
            button.BackgroundColor3 = Color3.fromRGB(255, 255, 255)
        end
    end
end

local function updateDisplay()
    resetItems()

    local invItems = scanItems()

    for toolName, toolData in pairs(invItems) do
        if invSlotData[slot1] == nil then

            invSlotData[slot1] = toolData

            slot1.CountDisplay.Text = toolData.Count
            slot1.Image = toolData.Items[1].TextureId

        elseif invSlotData[slot2] == nil then

            invSlotData[slot2] = toolData

            slot2.CountDisplay.Text = toolData.Count
            slot2.Image = toolData.Items[1].TextureId


        elseif invSlotData[slot3] == nil then

            invSlotData[slot3] = toolData

            slot3.CountDisplay.Text = toolData.Count
            slot3.Image = toolData.Items[1].TextureId

        elseif invSlotData[slot4] == nil then

            invSlotData[slot4] = toolData

            slot4.CountDisplay.Text = toolData.Count
            slot4.Image = toolData.Items[1].TextureId
        end
    end

    print(invSlotData)
end

task.spawn(function()
    while true do
        task.wait(1)
        updateDisplay()
    end
end)