I got the same problem. Are you using MySQL 5.x ?
I think it’s some sort of MySQL auto_imcrement issue similar to what was discussed here.
I was running 2.2.3 previously. Once I uploaded the 2.3 beta code, it redirected me from /wp-admin/ to /wp-admin/upgrade.php and told me an upgrade is needed. When I confirm, I get lots of these:
WordPress database error: [Duplicate entry ‘3’ for key 1]
INSERT INTO wp_terms (term_id, name, slug, term_group) VALUES (‘3’, ‘Virtualization’, ‘vmware-xen-etc-2’, ‘1’)
WordPress database error: [Duplicate entry ‘3-category’ for key 2]
INSERT INTO wp_term_taxonomy (term_id, taxonomy, description, parent, count) VALUES (‘3’, ‘category’, ”, ‘0’, ‘0’)
WordPress database error: [Duplicate entry ‘4’ for key 1]
