This message means that the article already contained that header line. Most likely a consequence of converting an email message into a news article by one means or another, without allowing for the differences in format and meaning of the header lines which are valid in the two cases. In general, the news server is available at all times. Reading news is usually possible while posting is disabled. While posting is disabled, attempts at posting will result in a message like the example above.
Newsreaders vary greatly in how informative they are when the news server refuses access.