How do I use SHELL to open app only once??

Something like this. You must know at least part of the window caption, which shouldn't be a problem because things like "Microsoft Word", "Microsoft Internet Explorer","Notepad" usually get appended to the end

In a Module

VB Code:

Private Declare Function EnumWindows Lib "user32" (ByVal lpEnumFunc As Long, ByVal lParam As Long) As Long
Private Declare Function SendMessage Lib "user32" Alias "SendMessageA" (ByVal hwnd As Long, ByVal wMsg As Long, ByVal wParam As Long, lParam As Any) As Long
Private Declare Function IsWindowVisible Lib "user32" (ByVal hwnd As Long) As Long
 
Private Const WM_GETTEXT = &HD
 
Private aCaptions() As String
Private lCount As Long
 
Public Function GetAllCaptions() As Variant
    lCount = 0
    Call EnumWindows(AddressOf EnumWindowsProc, 0&)
    If lCount Then ReDim Preserve aCaptions(lCount - 1)
    GetAllCaptions = IIf(lCount, aCaptions, Array())
End Function
 
Private Function EnumWindowsProc(ByVal hwnd As Long, ByVal lParam As Long) As Long
    Dim sBuffer As String * 260
    If IsWindowVisible(hwnd) Then
        ReDim Preserve aCaptions(lCount)
        aCaptions(lCount) = Left(sBuffer, SendMessage(hwnd, WM_GETTEXT, 260, ByVal sBuffer))
        If Len(Trim(aCaptions(lCount))) Then lCount = lCount + 1
    End If
    EnumWindowsProc = hwnd
End Function

Form Code:

VB Code:

Public Function IsOpen(strTitle As String) As Boolean
    
    Dim vCaps As Variant
    Dim lIndex As Long
    
    vCaps = GetAllCaptions()
 
    For lIndex = 0 To UBound(vCaps)
        If InStr(1, vCaps(lIndex), strTitle, vbTextCompare) Then
            IsOpen = True
            Exit Function
        End If
    Next
    
    IsOpen = False
 
End Function
 
Private Sub Command1_Click()
 
If IsOpen("Notepad") = False Then
    Shell "Notepad.exe", vbMaximizedFocus
End If
 
End Sub