Compare two lists and copy unique values to the second list

Hi Brumbot,
maybe so

Sub ert()
Dim x, i&
With Sheets("RM Input Table")
    x = .Range("C3", .Cells(Rows.Count, 3).End(xlUp)).Value
End With
With CreateObject("Scripting.Dictionary")
    .CompareMode = 1
    For i = 1 To UBound(x)
        .Item(x(i, 1)) = 1
    Next i
'    MsgBox .Count
    With Sheets("RM Input Table")
        x = .Range("A3", .Cells(Rows.Count, 1).End(xlUp)).Value
    End With
    For i = 1 To UBound(x)
        If .Exists(x(i, 1)) Then .Remove x(i, 1) Else .Item(x(i, 1)) = 1
    Next i
'    MsgBox .Count
    If .Count > 0 Then Sheets("RM Input Table").Cells(Rows.Count, 3).End(xlUp)(2).Resize(.Count).Value _
       = WorksheetFunction.Transpose(.keys)
End With
End Sub