WPF implemented Single Instance via mutex and activated the existed window via FindWindow from Microsoft.Win32; namespace

1.Remove StartUri="MainWindow.xaml" in App.xaml;

2.In the App.xaml.cs,overrive as below

using System;
using System.Collections.Generic;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Runtime.InteropServices;
using System.Threading.Tasks;
using System.Windows;
using Microsoft.Win32;

namespace WpfApp65
{
    /// <summary>
    /// Interaction logic for App.xaml
    /// </summary>
    public partial class App : Application
    {
        //lpWindowName,title name FindWindow(null,"WindowsTitleName");
        [DllImport("user32", CharSet = CharSet.Unicode)]
        static extern IntPtr FindWindow(string lpClassName, string lpWindowName);
        [DllImport("user32")]
        static extern IntPtr SetForegroundWindow(IntPtr hWnd);
         
        [DllImport("user32")]
        static extern bool IsIconic(IntPtr hWnd);

        [DllImport("user32")]
        static extern bool OpenIcon(IntPtr hWnd);
        protected override void OnStartup(StartupEventArgs e)
        { 
            bool isApplicationRunning = false;
            System.Threading.Mutex mutex = new System.Threading.Mutex
                (true, System.Diagnostics.Process.GetCurrentProcess().ProcessName,
                out isApplicationRunning);
            if(!isApplicationRunning)
            {
                MessageBox.Show("The app is running!", "Error", MessageBoxButton.OK, MessageBoxImage.Error);
                ActivateCurrentWindow();
                //The Shutdown() is indispensable else it will generate similar process every click
                Shutdown();
                return;
            } 
            else
            {
                MainWindow win = new MainWindow(Guid.NewGuid().ToString());
                win.Show();
            }
        }

        protected override void OnActivated(EventArgs e)
        {
            base.OnActivated(e);
        }

        private static void ActivateCurrentWindow()
        {
            //The second parameter is Windows Title
            var other = FindWindow(null, "Single Instance");
            if (other!=IntPtr.Zero)
            {
                SetForegroundWindow(other);
                if(IsIconic(other)) 
                {
                    OpenIcon(other);
                }
            }
        }
    }
}

3.Update the mainwindow as below, pass the guid value to validate whether it is the same instance.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace WpfApp65
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow(string str)
        {
            InitializeComponent();
            tb.Text = str;
        }
    }
}

The key part

 System.Threading.Mutex mutex = new System.Threading.Mutex
     (true, System.Diagnostics.Process.GetCurrentProcess().ProcessName,
     out isApplicationRunning);

//lpWindowName,title name FindWindow(null,"WindowsTitleName");
[DllImport("user32", CharSet = CharSet.Unicode)]
static extern IntPtr FindWindow(string lpClassName, string lpWindowName);