WordPress error while doing post

Hi
I was using wordpress software on my blog and when i tried to do a post, it got the error below:

WordPress database error: [Duplicate entry ‘2’ for key 1]
INSERT INTO wp_post2cat (post_id, category_id) VALUES (’35’, ‘5’)

Warning: Cannot modify header information – headers already sent by (output started at /home/secrets6/public_html/donaldlim/wp-includes/wp-db.php:133) in /home/secrets6/public_html/donaldlim/wp-includes/pluggable.php on line 329

It was working fine until I installed a few plugins. But I am unsure which plugins or something else caused the error.

The new post gets posted but it does not appear in any categories even though I have indicated which categories the new post shld be in. It get posted in the Uncategorize category although this category no longer exist on my blog.

Can some expert advise me on this please?
Thank.