【VBA】重複データをバックグラウンドで抽出する方法

Option Explicit
Sub ExtractDuplicateData()
    Dim ws As Worksheet, lastRow As Long, i As Long, j As Long
    Dim dict As Object
    Set ws = ThisWorkbook.Sheets("Sheet1")
    Set dict = CreateObject("Scripting.Dictionary")
    Application.ScreenUpdating = False
    
    ' 最終行を取得
    lastRow = ws.Cells(ws.Rows.Count, 1).End(xlUp).Row
    
    ' データの読み込みと重複チェック
    For i = 2 To lastRow
        If Not dict.exists(ws.Cells(i, 1)) Then
            dict.Add ws.Cells(i, 1), i
        End If
    Next i
    
    ' 重複データを抽出
    For j = 2 To lastRow
        If dict.exists(ws.Cells(j, 1)) And dict.Item(ws.Cells(j, 1)) <> j Then
            ws.Cells(j, 2).Value = "Duplicate"
        End If
    Next j
    
    Application.ScreenUpdating = True
End Sub