Messages that appear once in standard courier font and then are duplicated in another font (or in HTML) have been sent in a format known as "multipart/alternative." Unfortunately, Yahoo! Mail sometimes has difficulty interpreting messages sent in this format and shows the message twice, once in each format.

In the future, you might want to request that people sending you messages in "multipart/alternative" format use a different format, such as "plain text." Most email programs allow users to configure this option.