Worksheet Send Data Without Duplicate Value In EXcell VBA

Private Sub Worksheet_Change(ByVal Target As Range)
Dim i As Long
For i = 2 To Application.WorksheetFunction.CountA(Sheet1.Range("A:A"))
a = Application.WorksheetFunction.CountIf(Sheet1.Range("B:B"), _
Sheet1.Cells(i, 1).Value)
If a = 0 Then
Sheet1.Range("B1000000").End(xlUp).Offset(1, 0).Value = Sheet1.Cells(i, 1).Value
End If
Next i
End Sub