Allow only one instance of the app to run.

QueueSys/Program.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Linq;
 using System.Threading.Tasks;
 using System.Windows.Forms;
@@ -14,9 +15,33 @@ namespace QueueSys
         [STAThread]
         static void Main()
         {
+            if (PriorProcess() != null)
+            {
+
+                MessageBox.Show("Another instance of the app is already running.","Queue System",MessageBoxButtons.OK, MessageBoxIcon.Warning);
+                return;
+            }
+
             Application.EnableVisualStyles();
             Application.SetCompatibleTextRenderingDefault(false);
             Application.Run(new frmEmployeeView());
         }
+
+        public static Process PriorProcess()
+        // Returns a System.Diagnostics.Process pointing to
+        // a pre-existing process with the same name as the
+        // current one, if any; or null if the current process
+        // is unique.
+        {
+            Process curr = Process.GetCurrentProcess();
+            Process[] procs = Process.GetProcessesByName(curr.ProcessName);
+            foreach (Process p in procs)
+            {
+                if ((p.Id != curr.Id) &&
+                    (p.MainModule.FileName == curr.MainModule.FileName))
+                    return p;
+            }
+            return null;
+        }
     }
 }