Big Problem: Database Error

When I try and post, I get this:

WordPress database error: [Duplicate entry ‘0’ for key 1]
INSERT INTO wp_postmeta (post_id,meta_key,meta_value) VALUES (‘0′,’_encloseme’,’1′)

Warning: Cannot modify header information – headers already sent by (output started at /home/gunnmcn0/public_html/wp-includes/wp-db.php:102) in /home/gunnmcn0/public_html/wp-admin/post.php on line 60