string name = Process.GetCurrentProcess().ProcessName;
            int id = Process.GetCurrentProcess().Id;
            foreach (Process p in Process.GetProcesses())
            {
                if (name == p.ProcessName && id != p.Id)
                {
                    MessageBox.Show("你已经打开了此系统, 请先关闭！", string.Empty, MessageBoxButtons.OK, MessageBoxIcon.Warning);
                    return;
                }
            }

------解决方案--------------------

同意楼上
或者参考下
http://www.cnblogs.com/cyrix/articles/1874837.html