Do you prevent users from running two instances of your application?

In some cases, running two instances of an application at the same time may cause unexpected result. See this issue is solved via the code below on SSW Exchange Reporter:

try
{
	Process current = Process.GetCurrentProcess();
	Process[] processes = Process.GetProcessesByName( current.ProcessName);
			
	if ( processes.Length>1 )
	{
		DialogResult userOption = MessageBox.Show(Application.ProductName + " is already running on this machine. " + Environment.NewLine+Environment.NewLine + 		"Please click: "+Environment.NewLine+		  
		    " - 'Try again' to exit the other instance and try again, or "+Environment.NewLine+
		    " - 'Cancel' to exit now."+Environment.NewLine,
		    Application.ProductName+" "+(new Version(Application.ProductVersion)).ToString(2),
		  
		MessageBoxButtons.RetryCancel, MessageBoxIcon.Warning);
		switch(userOption)
		{
			case DialogResult.Cancel: return;
			  
			case DialogResult.Retry:
			foreach(Process currProcess in processes)
			{
				if ( currProcess.Id != current.Id)
				{
					currProcess.Kill();
				}
			}
			break;						
		}
	}	
}
catch (Exception ex)
{
	TracingHelper.Trace(null, Loggers.WindowsUILogger, TracingLevels.DEBUG, "Cannot get process information, Excpetion occured.", ex) ;

	DialogResult result = MessageBox.Show("Exchange Reporter cannot detect process information. This may be caused by disabled 'Performance Counter' on your machine. "+Environment.NewLine+
	"In such case, Exchange Reporter cannot ensure there is only one instance running. "+
	Environment.NewLine+
	"You may continue to run Exchange Reporter, however, please make sure you have only one instance of Exchange Reporter running. "+
	Environment.NewLine+
	"Multiple instances will cause unexpected behaviour. "+
	Environment.NewLine+Environment.NewLine+
	"Please click 'OK' to continue, or click 'Cancel' to quit."
	, Application.ProductName+" "+(new Version(Application.ProductVersion)).ToString(2),
	MessageBoxButtons.OKCancel,
	MessageBoxIcon.Warning);

	if ( result == DialogResult.Cancel)
	{					
		return;
	}
}

Code: Avoid running two instances of an application