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How to avoid loading a second instance of an application when the user already has one instance run

How to avoid loading a second instance of an application when the user already has one instance running. It also sets the focus to the first

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'Paste the following code into the code Module for Form1:
Option Explicit

Private Sub Form_Load()

   If App.PrevInstance Then
      ActivatePrevInstance
   End If

End Sub


'2) Add a Standard Module to the Project.
'3) Paste the following code into the module:

Option Explicit

Public Const GW_HWNDPREV = 3
Declare Function OpenIcon Lib "user32" (ByVal hwnd As Long) As Long
Declare Function FindWindow Lib "user32" Alias "FindWindowA" (ByVal lpClassName As String, ByVal lpWindowName As String) As Long
Declare Function GetWindow Lib "user32" (ByVal hwnd As Long, ByVal wCmd As Long) As Long
Declare Function SetForegroundWindow Lib "user32" (ByVal hwnd As Long) As Long

Sub ActivatePrevInstance()

   Dim OldTitle As String
   Dim PrevHndl As Long
   Dim result As Long

   'Save the title of the application.
   OldTitle = App.Title

   'Rename the title of this application so FindWindow
   'will not find this application instance.
   App.Title = "unwanted instance"

   'Attempt to get window handle using VB4 class name.
   PrevHndl = FindWindow("ThunderRTMain", OldTitle)

   'Check for no success.
   If PrevHndl = 0 Then

      'Attempt to get window handle using VB5 class name.
      PrevHndl = FindWindow("ThunderRT5Main", OldTitle)
   End If

   'Check if found
   If PrevHndl = 0 Then
        'Attempt to get window handle using VB6 class name
        PrevHndl = FindWindow("ThunderRT6Main", OldTitle)
   End If

   'Check if found
   If PrevHndl = 0 Then
      'No previous instance found.
      Exit Sub
   End If

   'Get handle to previous window.
   PrevHndl = GetWindow(PrevHndl, GW_HWNDPREV)

   'Restore the program.
   result = OpenIcon(PrevHndl)

   'Activate the application.
   result = SetForegroundWindow(PrevHndl)

   'End the application.
   End

End Sub