Macro To Flag Dupes In Next Column

[JADownie]

I have a column of data now (Col A) - Currently there are 3351 rows, but this can vary by week.

I am trying to write a macro now that will flag duplicate values in the next column over (Col B)

Example

Col A Col B
ABC
ABC Dupe
DEF
DEF Dupe
DEF Dupe
GHI
GHI Dupe
JKL


I have tried a couple of things now on my own, but I keep striking out! If anyone could be of assistance here today I would be greatly appreciative!

Thanks!!

 

[VBA Geek]

Sub Flaghtem()
    Dim l As Long
    l = Range("A:A").Find("*", , , , 1, 2).Row
    With Range("B1:B" & l)
        .Formula = "=IF(COUNTIF(A$1:A1, A1)>1, ""DUPE"", """")"
        .Value = .Value
    End With
End Sub