What happens is when you use rich text or html, you mail application sends the mail as a multipart/alternative, where the first part is the text as text/plain, and then the next part as text/rich or text/html.
Reading through the whole mail will then produce the same message twice, but most applications will choose the most complicated one it knows how to present. PINE for example will only show the first part, but Outlook will show the html or richtext