RemoteEvent Script [Must be in the NPC, Regular Script]

script.Parent.NPCTalk.OnServerEvent:connect(function(player)
    if not player.PlayerGui:FindFirstChild("") then -- Checks if the player already has the gui. Put your gui name between the quotation marks to the name of your GUI
        local gui = script.Parent:FindFirstChild(""):Clone() -- Change here too. Clones the gui
        gui.Parent = player.PlayerGui -- Clone the gui and put its parent to the player gui.
    end
end)