Truco de Visual Basic.NET - No permitir abrir el programa mas de una vez

Function PrevInstance() As Boolean
If UBound(System.Diagnostics.Process.GetProcessesByName(System.Diagnostics.Process.GetCurrentProcess.ProcessName)) > 0 Then
Return True
Else
Return False
End If
End Function

Usage:

If PrevExistance() = True Then Application.Exit()