editing posts: cannot modify header- duplicate entries for "key _" in 4.6cvs

When I try to edit a post I get a user error returned as for example which follows, although the post is actually published in edited form:

user error: Duplicate entry '18' for key 1
query: INSERT INTO copyright_node (nid, cpyid, children_show_notice) VALUES (18, 0, 0) in /home/mightyz/public_html/drupal/includes/database.mysql.inc on line 66.
warning: Cannot modify header information - headers already sent by (output started at /home/mightyz/public_html/drupal/includes/common.inc:385) in /home/mightyz/public_html/drupal/includes/common.inc on line 193.