This is not an html plugin, it is a VB/VBA function that has no preview, to see it working, you need to add it to your VBA project

If you don't have an idea of what is it, then this is not for you

Finds if there is at least one duplicated cell in a range of cells, returns TRUE if there is, FALSE if not
I needed this one for several projects and here it is

You would need to provide the range of cells to check (as range), it can be a column, a row, a 4x12 area or full sheet
Function IsDuplicated(Dupl_SheetRange As Range) As Boolean
Dim Dupl_AllRows, Dupl_Cell, Dupl_Count
Dupl_AllRows = Dupl_SheetRange.CurrentRegion.Cells.Count
IsDuplicated = False
For Each Dupl_Cell In Dupl_SheetRange.Cells
If Dupl_Cell.Value > "" Then
Dupl_Count = WorksheetFunction.CountIf(Dupl_SheetRange, Dupl_Cell.Value)
If Dupl_Count > 1 Then
IsDuplicated = True
Exit Function
End If
End If
End Function