Try:
Sub blah()
With Sheet2
Set SourceRng = .Range(.Range("A2"), .Cells(.Rows.Count, "C").End(xlUp))
End With
With Sheet1
Set Destn = .Cells(.Rows.Count, "A").End(xlUp).Offset(1)
Set ExistingPartNoRng = .Range(.Cells(7, "D"), Destn.Offset(-1, 3))
End With
For Each rw In SourceRng.Rows
If rw.Cells(3) = "NO" Then
If IsError(Application.Match(rw.Cells(2), ExistingPartNoRng, 0)) Then 'if unique?
Destn.Value = rw.Cells(1).Value 'copy index no.
Destn.Offset(, 3).Value = rw.Cells(2).Value 'copy part no.
Set Destn = Destn.Offset(1) 'update destination to next row down.
Set ExistingPartNoRng = ExistingPartNoRng.Resize(ExistingPartNoRng.Rows.Count + 1) 'update existing numbers range by adding bottomost cell.
End If
End If
Next rw
End Sub