One way:

Option Explicit
Sub Duplicate_Serial_Number()

   Dim myCell As Range
   Dim LastRow As Long
   Dim myRng As Range
   Dim myRngToShade As Range
   
   With ActiveSheet
       'clean up any previous shading
       .Cells.Interior.ColorIndex = xlNone
       LastRow = .Cells(.Rows.Count, "Q").End(xlUp).Row
       Set myRng = .Range("Q16:Q" & LastRow)

       If Application.Max(myRng) < 2 Then
           MsgBox "There is no duplicated serial number ", vbExclamation _
               + vbOKOnly, "No Duplicated Data"
       Else
           For Each myCell In myRng.Cells
               If myCell.Value > 1 Then
                   Set myRngToShade = .Range(.Cells(myCell.Row, "A"), _
                                                 .Cells(myCell.Row, "N"))
                   With myRngToShade.Interior
                       .ColorIndex = 6
                       .Pattern = xlSolid
                   End With
               End If
           Next myCell
       End If
   End With
       
End Sub

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Colors are pretty, but I like to use that extra column (Q) and then use
data|Filter|autofilter to see the duplicates.