Macro to find column with duplicates, for each of those check another column for duplicate

Wow, that's awesome Mick! After a couple hours staring at your code and looking up most of it in reference sheets. Breaking your code several times during trial and error because of tweaking it now works!

I ended up using the first macro and tweaked it. I also changes my requirements a little.

This is the code that I am using now:

Public Sub MG15May14()


Dim Rng         As Range
Dim Dn          As Range
Dim Twn         As String
Dim Frng        As Range
Dim LRng        As Range
Dim Q

Set Rng = Range(Range("C1"), Range("C" & Rows.Count).End(xlUp))
    With CreateObject("scripting.dictionary")
        .CompareMode = vbTextCompare

For Each Dn In Rng
    If Not Dn.Offset(, 1) = vbNullString And Not Dn.Offset(, 2) = vbNullString Then
        Twn = Dn & Dn.Offset(, 1) & Dn.Offset(, 2) & Dn.Offset(, 4) & Dn.Offset(, 10) & Dn.Offset(, 11) & Dn.Offset(, 12) & Dn.Offset(, 13) _
        & Dn.Offset(, 15) & Dn.Offset(, 16) & Dn.Offset(, 17)
            If Not .exists(Twn) Then
                .Add Twn, Array(Dn, Frng, LRng)
            Else
                Q = .Item(Twn)
                    If Q(2) Is Nothing Then
                        Set Q(2) = Dn
                    Else
                        If Q(1) Is Nothing Then
                            Set Q(1) = Q(2)
                        Else
                            Set Q(1) = Union(Q(1), Q(2))
                        End If
                        Set Q(2) = Dn
                    End If
            Q(0).EntireRow.Borders.ColorIndex = 5
            Q(0).EntireRow.Borders.Weight = xlThick
            Q(2).EntireRow.Borders.ColorIndex = 3
            Q(2).EntireRow.Borders.Weight = xlThick
            'If Not Q(1) Is Nothing Then Q(1).EntireRow.Interior.ColorIndex = 4 'xlNone
            '    .Item(Twn) = Q
        End If
    End If
Next
End With
'MsgBox "I just ran"
End Sub

And it's awesome I'm going to change the title of this post to 'solved'. Thank you so much Mick! I am not only happy that it works, but also that I learned a lot in the process .

I'm pleased you've been able to modify the code to your needs and thank you for the feedback
Regrds Mick