Add check so that only one instance can be open at a time

src/DesktopMagic/App.xaml.cs

@@ -1,4 +1,6 @@
-using System.Windows;
+using System;
+using System.Threading;
+using System.Windows;
 
 namespace DesktopMagic
 {
@@ -7,5 +9,30 @@ namespace DesktopMagic
     /// </summary>
     public partial class App : Application
     {
+        private Mutex _mutex;
+
+        public App()
+        {
+            // Try to grab mutex
+            bool createdNew;
+            _mutex = new Mutex(true, $"Stone_Red{DesktopMagic.MainWindow.AppName}", out createdNew);
+
+            //check if creating new was succesfull
+            if (!createdNew)
+            {
+                //Shutdown Aplication
+                Current.Shutdown();
+            }
+            else
+            {
+                // Add Event handler to exit event.
+                Exit += CloseMutexHandler;
+            }
+        }
+
+        protected virtual void CloseMutexHandler(object sender, EventArgs e)
+        {
+            _mutex?.Close();
+        }
     }
 }