Check Instance Of Application

Preferred Method:
Project Menu -> Properties -> Application -> Check Box: Make Single Instance Application

Non-Preferred Method:
This code will look in your task manager for the same name of your application, if it exists it will prevent another copy of it from running.

' Make sure they only have one instance of the application Open - Start
Private Sub CheckInstanceOfApp()
     Dim appProc() As Process
     Dim strModName, strProcName As String
     strModName = Process.GetCurrentProcess.MainModule.ModuleName
     strProcName = System.IO.Path.GetFileNameWithoutExtension(strModName)

     appProc = Process.GetProcessesByName(strProcName)
     If appProc.Length > 1 Then
          MessageBox.Show("There is an instance of this application running.", Application.ProductName)
          Application.Exit()
     Else
          'MessageBox.Show("There are no other instances running.", Application.ProductName)
     End If
End Sub
' Make sure they only have one instance of the application Open - Stop