How can I check for my running program instance?

How can I check for my running program instance? Lets say I am running program prog1.exe, If I run it again, I want to have a messaga saying that the program is already running, How can Chech for this. Any hint is appretiated. Thanks.

Code:

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Private Sub Form_Load()
    If App.PrevInstance = True Then
        Msgbox "This program is already running"
        Unload me
        Set form1 = nothing
    End If
End Sub

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