try this out

using System.Runtime.InteropServices;

#region Check for existing Instance
// get the name of our process
string proc=Process.GetCurrentProcess().ProcessName;
// get the list of all processes by that name
Process[] processes=Process.GetProcessesByName(proc);
// if there is more than one process...
if (processes.Length > 1)
{
MessageBox.Show("Already One Instance of this Application is
running.\nPlease check in the system Tray.\nThis Instance will Exit
now.");
// exit our process
Application.ExitThread();
                   
                }
                #endregion