The Jargon File (version 4.4.7, 29 Dec 2003):

cross-post
 vi.

    [Usenet; very common] To post a single article simultaneously to several
    newsgroups. Distinguished from posting the article repeatedly, once to each
    newsgroup, which causes people to see it multiple times (which is very bad
    form). Gratuitous cross-posting without a Followup-To line directing
    responses to a single followup group is frowned upon, as it tends to cause
    followup articles to go to inappropriate newsgroups when people respond
    to various parts of the original posting.


The Free On-line Dictionary of Computing (30 December 2018):

cross-post

   [Usenet] To post a single article simultaneously to several
   newsgroups.  Distinguished from posting the article
   repeatedly, once to each newsgroup, which causes people to see
   it multiple times (which is very bad form).  Gratuitous
   cross-posting without a Followup-To line directing responses
   to a single followup group is frowned upon, as it tends to
   cause followup articles to go to inappropriate newsgroups
   when people respond to only one part of the original posting.

   [Jargon File]