Whenever I reply to a message, the reply as well as the original appears in my inbox. I thought it might be related to Threaded Replies, but that is set in Sort>View to "Unthreaded."

How do I stop this? I'd like to keep the original message I rec'd. And if I need to see my reply I can go to 'Sent' folder. I don't need each and every message in a back-and-forth filling my inbox.