Prevent multiple logins using command line args, if statement checks, C#

I am currently writing a login method that keeps track of a logged in user by reading the contents of credentials saved in a text file called "accounts.txt".

The method works fine up to this point however I dont understand how I could make a simple check to prevent multiple logins. I am using command line arguments to input a string that logs in a user.

Example of this when input by command line is "login tom C#password" with "login" being passed as the command parameter, "tom" as param1, and "C#password" as the param2.

Ideally the method should check as an if statement that prevents a double login until the user inputs a "logout" command after "login" has already been used by command line args.

I'm fairly new to c#, any kind of advice is appreciated

 protected void login(string command, string param1, string param2) // string command "login" string username, string password
        {
            // login command
            // Needs to check check accounts.txt file for username and password.
            // if username and password exists, provide login message/else failed login
            // NO two logins at the same time until logout command is provided after the first login
            // checks accounts.txt file if username already exists and if there is a match, if not a prompt says login has failed

            var logins = File.ReadAllLines(@"C:\\Files\\accounts.txt");
            if (command == "login")
            {
                if (logins.Any(l => l == string.Format("{0} {1}", param1, param2)))
                    Console.WriteLine("Login {0} ", param1);
                string path1 = "C:\\Files\\audit.txt";
                using (StreamWriter sw1 = File.AppendText(path1))
                {
                    sw1.WriteLine("User " + param1 + " logged in");
                }
            }
            else
                Console.WriteLine("Login Failed: Invaild username or password");
                }

            //checks if more than one user logged in

            if (check for double login command)
            {
                Console.WriteLine("Login Failed: simultaneous login not permitted");
                string path2 = "C:\\Files\\audit.txt";
                using (StreamWriter sw2 = File.AppendText(path2))
                {
                sw2.WriteLine("Login Failed: simultaneous login not permitted");
                }
            }
            Console.Read();
        }

logout is a simple method that records if a user has logged out

 protected void logout(string command, string param1)
        {
            // remove current username and password from memory
        var logout = File.ReadAllLines(@"C:\\Files\\accounts.txt");
        if (command == "logout")
        {
            if (logout.Any(l => l == string.Format("{0} {1}", param1, param2)))
                Console.WriteLine("Logout {0} ", param1);
            string path1 = "C:\\Files\\audit.txt";
            using (StreamWriter sw1 = File.AppendText(path1))
            {
                sw1.WriteLine("User " + param1 + " logged out");
            }
        }
        else
            Console.WriteLine("Logout Failed");

        Console.Read();
    }