Code:
Sub test()
Dim i%, k%
Cells.Interior.ColorIndex = xlNone
For i = 1 To Range("C65536").End(xlUp).Row-1
For k = i+1 To Range("C65536").End(xlUp).Row
If Cells(i, 3)= Cells(k, 3) Then
Rows(i).Interior.Color = vbRed
Rows(k).Interior.Color = vbRed
End If
Next k
Next i
End Sub
A+