I recorded a macro and I tried to autofill column C with the COUNTIF function all the way down until B column has their last cell value. It doesn't work with new data so I tried to edit the macro and replace it with Range("D2:D") but that doesn't work.
Sub COUNTIF()
'
' COUNTIF Macro
'
'
Range("D2").Select
ActiveCell.FormulaR1C1 = "=COUNTIF(C[-3],RC[-3])>1"
Range("D2").Select
Selection.AutoFill Destination:=Range("D2:D")
End Sub