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Option Explicit
'KERNEL32
Private Declare Function CreateSemaphoreW Lib "kernel32.dll" (ByVal lpSemaphoreAttributes As Long, ByVal lInitialCount As Long, ByVal lMaximumCount As Long, ByVal lpName As Long) As Long
 
Private Const ERROR_ALREADY_EXISTS  As Long = 183&
 
Public Function bWasIOpened(ByVal lpName As String) As Boolean
    bWasIOpened = (CreateSemaphoreW(0&, 0&, 1&, StrPtr(lpName)) > 0) And (Err.LastDllError = ERROR_ALREADY_EXISTS)
End Function

If App.PrevInstance = True Then End

Private Sub Form_Load()
    If bWasIOpened("Cualquier_Cosa") = True Then End
End Sub