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Determine if a workbook is already open

 1999-12-20    Workbooks    0    216

The function below can be used to determine if a workbook is open or not: 

Function WorkbookOpen(WorkBookName As String) As Boolean
' returns TRUE if the workbook is open
    WorkbookOpen = False
    On Error GoTo WorkBookNotOpen
    If Len(Application.WorkBooks(WorkBookName).Name) > 0 Then
        WorkbookOpen = True
        Exit Function
    End If
WorkBookNotOpen:
End Function
Example: 
If Not WorkbookOpen("MyWorkbookName.xls") Then 
    Workbooks.Open "MyWorkbookName.xls"
End If
You can use another macro to determine if a workbook is opened by another user or process.