Default to preventing multiple instances

Allow multiple instances if overridden with command line parameter
Allow starting minimised to tray with command line parameter

ntfysh_client/Program.cs

@@ -1,25 +1,46 @@
 using System;
-using System.Collections.Generic;
+using System.Diagnostics;
+using System.IO;
 using System.Linq;
-using System.Threading.Tasks;
+using System.Reflection;
 using System.Windows.Forms;
 
 namespace ntfysh_client
 {
     static class Program
     {
-        private static readonly NotificationListener NotificationListener = new NotificationListener();
+        private static readonly NotificationListener NotificationListener = new();
         
         /// <summary>
         /// The main entry point for the application.
         /// </summary>
         [STAThread]
-        static void Main()
+        private static void Main(string[] args)
         {
+            args = args.Select(a => a.ToLower()).ToArray();
+
+            if (args.Contains("-h") || args.Contains("--help"))
+            {
+                MessageBox.Show("Help:\n    -h\n    --help\n\nStart in tray:\n    -t\n    --start-in-tray\n\nAllow multiple instances:\n    -m\n    --allow-multiple-instances", "Help Menu", MessageBoxButtons.OK, MessageBoxIcon.Information);
+                return;
+            }
+
+            bool startInTray = args.Contains("-t") || args.Contains("--start-in-tray");
+            bool allowMultipleInstances = args.Contains("-m") || args.Contains("--allow-multiple-instances");
+
+            if (Process.GetProcessesByName(Path.GetFileNameWithoutExtension(Assembly.GetEntryAssembly()!.Location)).Length > 1)
+            {
+                if (!allowMultipleInstances)
+                {
+                    MessageBox.Show("Another instance is already running.\n\nUse -m or --allow-multiple-instances if you wish to start a second duplicate instance.\n\nThis instance will now close.", "Multiple Instances", MessageBoxButtons.OK, MessageBoxIcon.Warning);
+                    return;
+                }
+            }
+            
             Application.SetHighDpiMode(HighDpiMode.SystemAware);
             Application.EnableVisualStyles();
             Application.SetCompatibleTextRenderingDefault(false);
-            Application.Run(new MainForm(NotificationListener));
+            Application.Run(new MainForm(NotificationListener, startInTray));
         }
     }
 }