How do i clone a tool in my backpack?

You can either use the PlayerAdded event from game.Players or fire a RemoteEvent or RemoteFunction with a localscript.

Option 1: PlayerAdded

game.Players.PlayerAdded:Connect(function(plr)
  local backpack = plr:WaitForChild("Backpack");
  local tool = game.ReplicatedStorage.Tool:Clone(); -- Change this to the tool location
  local ctool = tool:Clone();
  ctool.Parent = backpack;
end

Option 2: A: RemoteEvent

game.ReplicatedStorage.RemoteEvent.OnServerEvent:Connect(function(plr) -- change to the location of your RemoteEvent.
local backpack = plr:WaitForChild("Backpack");
  local tool = game.ReplicatedStorage.Tool:Clone(); -- Change this to the tool location
  local ctool = tool:Clone();
  ctool.Parent = backpack;
end)

Option 2: B: RemoteFunction

game.ReplicatedStorage.RemoteFunction.OnServerInvoke = function(plr)
local backpack = plr:WaitForChild("Backpack");
  local tool = game.ReplicatedStorage.Tool:Clone(); -- Change this to the tool location
  local ctool = tool:Clone();
  ctool.Parent = backpack;
  return true; -- This is really the only reason you'd use RemoteFunction ;- to return something after it executes to the client who called it.
end

EDIT: I just saw that you wanted to clone the tool in the player's backpack, in which case you'd use Option 2 (either A or B, whichever you think would help more). You'd want to clone the tool in backpack instead of the one in ReplicatedStorage.

If you want to clone a tool in your backpack, first we need to get the local player since the player's backpack is locally based.

player = game.Players.LocalPlayer

Next, we need to actually type the script that clones the tool in your backpack and puts it in your backpack again.

player.Backpack.M9:Clone().Parent = player.Backpack

So in the end, this is the final script:

player = game.Players.LocalPlayer

player.Backpack.M9:Clone().Parent = player.Backpack