If/Then Loop to find Matches in Column

Ok, this one shouldn't be that hard.

I'm looking to go down column A, and find all of the doubled (or tripled, quad... ect) values. I originally would manually insert a cloumn, and write an "if" function to place an X if the cell R[-1]C[-1] was equal to the cell RC[-1]. I'd then run the macro to delete them. However, I want to have it all automated, so i'm trying to do that here in the top of the macro here.

(The if/then loop)

It doesn't work . The erasing of rows that have X's works fine. It's just that I cant get the macro to place X's.

Sub Example()

    Dim DeleteValue As String
    Dim rng As Range
    Dim calcmode As Long

    With Application
        calcmode = .Calculation
        .Calculation = xlCalculationManual
        .ScreenUpdating = False
    End With
    
    Workbooks("Example").Worksheets("Sheet1").Columns(2).Insert
    

    Do While IsEmpty(ActiveCell.Offset(0, -1)) = False
        If ActiveCell.FormulaR3C2 = "(R[-1]C[-1])" Then
        ActiveCell = "X"
        End If
    ActiveCell.Offset(1, 0).Select
    Loop

    DeleteValue = "X"

    With ActiveSheet

        .AutoFilterMode = False

        .Range("B1:B" & .Rows.Count).AutoFilter Field:=1, Criteria1:=DeleteValue

        With .AutoFilter.Range
            On Error Resume Next
            Set rng = .Offset(1, 0).Resize(.Rows.Count - 1, 1) _
                      .SpecialCells(xlCellTypeVisible)
            On Error GoTo 0
            If Not rng Is Nothing Then rng.EntireRow.Delete
        End With

        .AutoFilterMode = False
    End With

    With Application
        .ScreenUpdating = True
        .Calculation = calcmode
    End With

End Sub

Thanks,


`Ryan