Your message is sent to the original author as well as to the mailing list. That usually means the author receives two copies. Even worse, his address will be in the new To line. It will be copied into subsequent followups, so he will receive two copies of future messages. After a long chain of followups there can be dozens of extra addresses.
Archive space is wasted.
The To line becomes unreadable. Some software breaks. Your follow-up function probably eliminates duplicate addresses. But it can't detect the duplicate here. Perhaps the original author isn't on the mailing list, and should receive a copy.