Sub ertert()
Dim x, i&, j&
With Sheets("Все_клиенты")
     x = .Range("A1:B" & .Cells(Rows.Count, 1).End(xlUp).Row).Value
End With
With CreateObject("Scripting.Dictionary")
     .CompareMode = 1
     For i = 1 To UBound(x)
         .Item(x(i, 1)) = Empty
     Next i
     With Sheets("Клиенты_на_рассмотрении")
         x = .Range("A1:B" & .Cells(Rows.Count, 1).End(xlUp).Row).Value
     End With
     For i = 1 To UBound(x)
         If Not .Exists(x(i, 1)) Then
             .Item(x(i, 1)) = Empty
             j = j + 1: x(j, 1) = x(i, 1): x(j, 2) = x(i, 2)
         End If
     Next i
End With
With Sheets("Все_клиенты")
     If j > 0 Then .Cells(Rows.Count, 1).End(xlUp)(2).Resize(j, 2).Value = x
     .Activate
End With
End Sub