If the data in Column C (the column I am assuming you want to check for doubles... change the highlighted C's to the actual column letter for your data) is all text, that is, no formulas, then here is one more macro for you to try (this one uses no loops and should be fast... no promises on that though)...
Code:
Sub MarkDuplicates()
  Dim Addr As String
  Addr = "C1:C" & Cells(Rows.Count, "C").End(xlUp).Row
  Range(Addr) = Evaluate("IF(COUNTIF(" & Addr & "," & Addr & ")>1,""=""&" & Addr & "," & Addr & ")")
  On Error Resume Next
  Range(Addr).SpecialCells(xlFormulas).Interior.ColorIndex = 6
  Range(Addr).Replace "=", "", xlPart
End Sub
By the way, if your data column does contain formulas, I can modify the code to account for that... just let me know.