A drupal site that's run fine for quite a while just started producing this error. Any ideas what the cause is? I flushed the cache, but that didn't repair it. 

user error: Duplicate entry ‘menu:0:en’ for key 1
query: INSERT INTO cache (cid, data, created, expire, headers) VALUES (‘menu:0:en’, ‘a:3:{s:10:\”path index\”;a:227:{s:16:\”admin/aggregator\”;s:1:\”2\”;s:26:\”admin/aggregator/edit/feed\”;i:-2;s:30:\”admin/aggregator/edit/category\”;i:-3;s:23:\”admin/aggregator/remove\”;i:-4;s:23:\”admin/aggregator/update\”;i:-5;s:21:\”admin/aggregator/list\”;i:-6;s:25:\”admin/aggregator/add/feed\”;i:-7;s:29:\”admin/aggregator/add/category\”;i:-8;s:10:\”aggregator\”;s:1:\”3\”;s:18:\”aggregator/sources\”;s:1:\”4\”;s:21:\”aggregator/categories\”;s:1:\”5\”;s:15:\”aggregator/opml\”;i:-12;s:6:\”banner\”;i:-13;s:7:\”banners\”;s:1:\”6\”;s:12:\”admin/banner\”;s:1:\”7\”;s:12:\”banners/view\”;i:-16;s:11:\”banners/add\”;i:-17;s:17:\”admin/banner/list\” in /home/httpd/vhosts/isna.org/httpdocs/includes/database.mysql.inc on line 66.

Environment