Public Sub Main()
If PrevInstance() Then Exit Sub

' continue with your application
UserName = Environ("UserName")
ComputerName = Environ("COMPUTERNAME")

End Sub

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