Duplicate comments from the same user could be inserted to the database

[caseqiu]

In file wp_includes/comment.php function wp_new_comment, it first checks if the comment content submitted by the user already exists in the database (in wp_allow_comment) and then inserts the comment. When race condition happens, it could happen that the server handles two requests of submitting a comment with the same content from the same user and both pass the wp_allow_comment check, resulting in duplicate comments from the same user is inserted to the database.