SQL Error "duplicate entry..." on edit of panes

I keep getting the following error:

user warning: Duplicate entry '28' for key 1 query: panels_save_display INSERT INTO panels_pane (pid, did, panel, type, subtype, configuration, cache, access, position) VALUES (28, 9, 'section3', 'block', '', 'a:4:{s:6:\"module\";s:5:\"block\";s:5:\"delta\";s:2:\"12\";s:14:\"override_title\";i:0;s:19:\"override_title_text\";s:0:\"\";}', 'a:0:{}', '', 0) in /home/shezoom/public_html/test/includes/database.mysql.inc on line 172.

It doesn't happen every time, but it happens often, when I am updating content in custom content panels. I can't tell offhand whether it's actually having an effect, since my changes seem to be saving.