bueno suena simple pero no he podido dar con la solución a esto.. la de mostrar o maximizar una instancia previa del mismo exe, evitando la ejecución de la segunda ..

para evitar ejecución del mismo exe 2 veces uso la función api CreateMutex :

Option Explicit
 
'Code by Adam Verwijs
Const ERROR_ALREADY_EXISTS = 183&
Private Declare Function CreateMutex Lib "kernel32" Alias "CreateMutexA" (lpMutexAttributes As Any, ByVal bInitialOwner As Long, ByVal lpName As String) As Long
Private Declare Function ReleaseMutex Lib "kernel32" (ByVal hMutex As Long) As Long
Private Declare Function CloseHandle Lib "kernel32" (ByVal hObject As Long) As Long
 
'Private Declare Function IsIconic Lib "user32" (ByVal hwnd As Long) As Long
Private Declare Function SetForegroundWindow Lib "user32" (ByVal hwnd As Long) As Long
Private Declare Function FindWindow Lib "user32" Alias "FindWindowA" (ByVal lpClassName As String, ByVal lpWindowName As String) As Long
Private Declare Function ShowWindow Lib "user32" (ByVal hwnd As Long, ByVal nCmdShow As Long) As Long
 
Const SW_NORMAL = 1
 
Dim ventana As Long
 
Private Sub Form_Load()
    Dim hMutex As Long
    'Try to create a new Mutex
    hMutex = CreateMutex(ByVal 0&, 1, App.Title)
    'Did the mutex already exist?
    If (Err.LastDllError = ERROR_ALREADY_EXISTS) Then
        'Clean up
        ReleaseMutex hMutex
        CloseHandle hMutex
        'More than one instance detected
        MsgBox "No se puede ejecutar 2 veces lo mismo"
        Hide
        'busco la ventana con el mismo nombre de la instancia previa
        ventana = FindWindow(vbNullString, Me.Caption)
        'llamo al hwnd de la ventana encontrada
        Call SetForegroundWindow(ventana)
        'muestro ala ventana mediante su hwnd
        Call ShowWindow(ventana, SW_NORMAL)
        End
 
    End If
End Sub
 
 

ayuda y sugerencias.. seran bienvenidas..  

Edit: solucionado.

Option Explicit
 
'Code by Adam Verwijs
Const ERROR_ALREADY_EXISTS = 183&
Private Declare Function CreateMutex Lib "kernel32" Alias "CreateMutexA" (lpMutexAttributes As Any, ByVal bInitialOwner As Long, ByVal lpName As String) As Long
Private Declare Function ReleaseMutex Lib "kernel32" (ByVal hMutex As Long) As Long
Private Declare Function CloseHandle Lib "kernel32" (ByVal hObject As Long) As Long
 
Private Declare Function SetForegroundWindow Lib "user32" (ByVal hwnd As Long) As Long
Private Declare Function FindWindow Lib "user32" Alias "FindWindowA" (ByVal lpClassName As String, ByVal lpWindowName As String) As Long
Private Declare Function ShowWindow Lib "user32" (ByVal hwnd As Long, ByVal nCmdShow As Long) As Long
 
Const SW_NORMAL = 1
 
Dim ventana As Long
 
Private Sub Form_Load()
 
    Dim hMutex As Long
    Me.Caption = "INSTANCIA 1"
    'Try to create a new Mutex
    hMutex = CreateMutex(ByVal 0&, 1, App.Title)
    'Did the mutex already exist?
    If (Err.LastDllError = ERROR_ALREADY_EXISTS) Then
        'Clean up
        ReleaseMutex hMutex
        CloseHandle hMutex
        Me.Caption = "INSTANCIA 2"
        Hide
        'busco la ventana con el mismo nombre de la instancia previa
        ventana = FindWindow(vbNullString, "INSTANCIA 1")
        'llamo al hwnd de la ventana encontrada
        Call SetForegroundWindow(ventana)
        'muestro ala ventana mediante su hwnd
        Call ShowWindow(ventana, SW_NORMAL)
        End
    End If
 
End Sub
 

Saludos.

Creo que haci deberia ser no?

Option Explicit
 
'Code by Adam Verwijs
Const ERROR_ALREADY_EXISTS = 183&
Private Declare Function CreateMutex Lib "kernel32" Alias "CreateMutexA" (lpMutexAttributes As Any, ByVal bInitialOwner As Long, ByVal lpName As String) As Long
Private Declare Function ReleaseMutex Lib "kernel32" (ByVal hMutex As Long) As Long
Private Declare Function CloseHandle Lib "kernel32" (ByVal hObject As Long) As Long
 
'Private Declare Function IsIconic Lib "user32" (ByVal hwnd As Long) As Long
Private Declare Function SetForegroundWindow Lib "user32" (ByVal hwnd As Long) As Long
Private Declare Function FindWindow Lib "user32" Alias "FindWindowA" (ByVal lpClassName As String, ByVal lpWindowName As String) As Long
Private Declare Function ShowWindow Lib "user32" (ByVal hwnd As Long, ByVal nCmdShow As Long) As Long
 
Const SW_NORMAL = 1
 
Dim ventana As Long
 
Private Sub Form_Load()
    Hide
    Dim hMutex As Long
    Dim Caption2 As String, AntiInstancia As String
    Caption2 = "Hola Mundo"
    Caption = Caption2
    'Try to create a new Mutex
    AntiInstancia = "Mutexrecor"
    hMutex = CreateMutex(ByVal 0&, 1, AntiInstancia)
    'Did the mutex already exist?
    If (Err.LastDllError = ERROR_ALREADY_EXISTS) Then
        'Clean up
        ReleaseMutex hMutex
        CloseHandle hMutex
        'More than one instance detected
        'busco la ventana con el mismo nombre de la instancia previa
        caption="Error: Doble Instancia"
        ventana = FindWindow(vbNullString, Caption2)
        'llamo al hwnd de la ventana encontrada
        Call SetForegroundWindow(ventana)
        'muestro ala ventana mediante su hwnd
        Call ShowWindow(ventana, SW_NORMAL)
        End
    End If
    Show
End Sub
 
 

gracias por sus respuestas, lo del artificio de cambiarle el nombre a la ventana de la segunda instancia me lo habian comentado antes, pero ahora ya me lo confirmaron y es mas hasta adecuaron el code..

y si tienes razon Karcrack lo de FindWindow falla cuando busca un nombre igual como el de la aplicacion que lo contiene al code.

Edit: lo de Initialize tampoco da .. hasta intente con sub main con el mismo resultado.

Saludos.