Db error when dealing with blogroll or categories

anytime I update my blogroll, or move a post from one category to another I get something like this:

WordPress database error: [Duplicate entry ’47’ for key 1]
INSERT INTO wp_term_relationships (object_id, term_taxonomy_id) VALUES (’47’, ’21’)

Warning: Cannot modify header information – headers already sent by (output started at /home/ubuntutu/public_html/wp-includes/wp-db.php:160) in /home/ubuntutu/public_html/wp-includes/pluggable.php on line 390

I have tried disabling all plugins and I am using a theme from the approved list. Please someone point me on the right direction.