Sub Demo()
Application.ScreenUpdating = False
Dim i As Long
With ActiveDocument
  For i = 2 To .Paragraphs.Count
    With .Paragraphs(i)
      If .Range.Text = .Previous.Range.Text Then
        .Previous.Range.HighlightColorIndex = wdYellow
      End If
    End With
  Next
End With
Application.ScreenUpdating = True
End Sub