Same level for all players?

There are a lot of problems with your code. I'm going to go through it line by line and refactor it. This may not fix anything, but it will run a lot better.

local ds = game:GetService("DataStoreService"):GetDataStore("dataone")

function PlayerAdded(newPlayer) --This function is called when a Player gets added. Why is it called 'CharacterRemoving'?

--I'm not really sure what to do with this block of code. It definitely doesn't go inside the PlayerAdded function. What are you trying to do with this?
--[[    p = game.Players:GetPlayers()
    for i = 1, #p do
        s = Instance.new("StringValue", p[i].Character)
        s.Name = "Playing"
    end]]

    local stats = Instance.new("IntValue", newPlayer) --We can use this second argument to set the Parent on the same line.
    stats.Name = "leaderstats"

     local coins = Instance.new("IntValue", stats) --ALL of these properties are now local! If they aren't, they will overwrite each other and we don't want that.
    coins.Name = "Coins"

    local tokens = Instance.new("IntValue", stats)
    tokens.Name = "Tokens"

    local xp = Instance.new("IntValue", stats)
    xp.Name = "XP"

    local levels = Instance.new("IntValue", stats)
    levels.Name = "Levels"

    local coinKey = ds:GetAsync("Key" .. newPlayer.Name) --GetAsync called more than once is wasteful. You have a limit to the number of times you can call this per minute!
    if coinKey then
        coins.Value = coinKey
    else
        coins.Value = 0
    end

    local tokensKey = ds:GetAsync("Key1" .. newPlayer.Name)
    if  tokensKey then
        tokens.Value = tokensKey
    else
        tokens.Value = 0
    end
    local xpKey = ds:GetAsync("Key2" .. newPlayer.Name)
    if xpKey then
        xp.Value =xpKey
    else
        xp.Value = 0
    end
    local levelsKey = ds:GetAsync("Key3" .. newPlayer.Name)
    if  then
        levels.Value = ds:GetAsync("Key3" .. newPlayer.Name)    
    else
        levels.Value = 1
    end

    --This while loop was repeatedly creating these function connections once a second. Changed events will fire every single time they are fired!
        coins.Changed:connect(function(save)
            ds:SetAsync("Key" .. newPlayer.Name, save)
        end)
        tokens.Changed:connect(function(save)
            ds:SetAsync("Key1" .. newPlayer.Name, save)
        end)

    local xpDeb = false
        xp.Changed:connect(function() --I made this function control saving of both levels and xp, based on your formula below.
        if xpDeb then return end
        xpDeb = true
        while xp.Value >= 100 do
            xp.Value = xp.Value - 100
            levels.Value = levels.Value + 1
        end
            ds:SetAsync("Key2" .. newPlayer.Name, xp.Value)
        ds:SetAsync("Key3" .. newPlayer.Name, levels.Value)
        xpDeb = false
        end)

    local function CharacterAdded(newCharacter) --NEVER use the same global function name twice. This can cause unforeseeable bugs and errors!
        --This function had to be moved inside here so that trying to access the various ValueObjects that are now local don't cause any errors.
        --We already loaded this above! No need to do it again.
        --levels.Value = ds:GetAsync("Key3" .. newPlayer.Name)
        wait(1.1) --No need for two wait statements

        local gui = Instance.new("BillboardGui") --This could have been your error! When this function was called more than once, the global `gui` variable got overwritten. Setting it to be a local variable makes sure that your code works as intended: only editing THIS BillboardGui.
        gui.Parent = newPlayer.Character.Head
        gui.Adornee = newPlayer.Character.Head
        gui.Size = UDim2.new(3,0,2,0)
        gui.StudsOffset = Vector3.new(0,2,0)

        local text = Instance.new("TextLabel")
        text.Text = "Level " .. newPlayer.leaderstats.Levels.Value
        text.Size = UDim2.new(1.25,0,0.5,0)
        text.Position = UDim2.new(-0.125,0,-0.25,0)
        text.BackgroundTransparency = 1
        text.Parent = gui
        text.FontSize = ("Size36")
        text.Font = ("ArialBold")
        text.TextColor3 = Color3.new(255, 255, 0)
        text.TextStrokeColor3 = Color3.new(0, 0, 0)
        text.TextStrokeTransparency = 0

        for _, wep in ipairs(game.Lighting.WepsGroup:GetChildren()) do --A generic `for` is better here than a numeric.
            wep:Clone().Parent = newPlayer.Backpack
        end
    end
    newPlayer.CharacterAdded:connect(CharacterAdded)
end
game.Players.PlayerAdded:connect(PlayerAdded)

--All the lines after this are completely unnecessary.

If this still errors, please let me know where.