あなたの投稿があなたが使用しようとしたことを示唆しているように、数式でそれを行うには
=IF(ISNA(MATCH(A2,B:B,0)),"","duplicate")
VBAソリューションについては、これを試してください
Sub CheckForDups()
Dim rSource As Range
Dim rCompare As Range
Dim rResult As Range
Dim vSource As Variant
Dim vComapre As Variant
Dim vResult As Variant
Dim sh As Worksheet
Dim i As Long
Set sh = ActiveSheet
Set rSource = Application.Intersect(sh.UsedRange, sh.Columns(1))
Set rCompare = Application.Intersect(sh.UsedRange, sh.Columns(2))
Set rResult = Application.Intersect(sh.UsedRange, sh.Columns(3))
vSource = rSource
vComapre = rCompare
rResult.Clear
vResult = rResult
For i = 2 To UBound(vSource, 1)
If Not IsError(Application.Match(vSource(i, 1), rCompare, 0)) Then
vResult(i, 1) = "duplicate"
End If
Next
rResult = vResult
End Sub