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. To also lets people redirect discussions away from inappropriate mailing lists. You can omit it if you're using the default.