This will get you started.
Code:
Sub ShowDups()
Dim aPar As Paragraph, sStart As String
On Error Resume Next
For Each aPar In ActiveDocument.Paragraphs
If aPar.Range.Characters(2).Text = vbTab Then
If Left(aPar.Range.Text, 2) = Left(aPar.Next.Range.Text, 2) Then
aPar.Next.Range.HighlightColorIndex = wdBrightGreen
End If
End If
Next aPar
End Sub
I put the error resume in because the loop would fail when looking for the paragraph after the last paragraph (and I couldn't be bothered complicating the code to deal with that).