A列とB列を比較して同じデータがあれば「1回だけ」B列の該当データに色を付けるようなマクロや関数をお教えいただきたいです。

Sub main()

Dim a As Long
Dim b As Long


For a = 1 To Rows.Count
    If Cells(a, "A") = "" Then Exit For
    For b = 1 To Rows.Count
        If Cells(b, "B") = "" Then Exit For
        If Cells(a, "A") = Cells(b, "B") Then
            Cells(b, "B").Interior.ColorIndex = 6
            Exit For
        End If
    Next b
Next a

End Sub