Compare 2 columns and mark the duplicates

Ok, title changed

Hello powpowninjastar,

Thank you for changing the title. You can change the sheet names and the starting cells in the code to match what you will be using. Copy this code to a standard VBA module in your workbook.

Sub CompareColumns()

  Dim Cell As Range
  Dim DSO As Object
  Dim Key As String
  Dim Item As Long
  Dim Rng As Range
  Dim RngEnd As Range
  Dim Sht1 As Worksheet
  Dim Sht2 As Worksheet
  
    Set DSO = CreateObject("Scripting.Dictionary")
    DSO.CompareMode = vbTextCompare
    
    Set Sht1 = Worksheets("Sheet1")
    Set Sht2 = Worksheets("Sheet2")
    
      Set Rng = Sht1.Range("A1")
      Set RngEnd = Sht1.Cells(Rows.Count, Rng.Column).End(xlUp)
      Set Rng = IIf(RngEnd.Row < Rng.Row, Rng, Sht1.Range(Rng, RngEnd))
      
      For Each Cell In Rng
        Key = Trim(Cell.Text)
        If Key <> "" Then
          If Not DSO.Exists(Key) Then DSO.Add Key, 0
        End If
      Next Cell
      
      Set Rng = Sht2.Range("A1")
      Set RngEnd = Sht2.Cells(Rows.Count, Rng.Column).End(xlUp)
      Set Rng = IIf(RngEnd.Row < Rng.Row, Rng, Sht2.Range(Rng, RngEnd))
      
      For Each Cell In Rng
        Key = Trim(Cell.Text)
        If Key <> "" Then
          If DSO.Exists(Key) Then Cell.Offset(0, 1) = "x"
        End If
      Next Cell
      
    Set DSO = Nothing
    
End Sub