Continuous alter table error in logs

I've just upgraded a site to 2.8.1, and seem to be getting things chugging along more reliably now, but I am seeing a large number of errors like below in my errors log:

WordPress database error Duplicate column name 'comment_subscribe' for query ALTER TABLE wp_4_comments ADD COLUMN comment_subscribe enum('Y','N') NOT NULL default 'N' made by db_upgrade_check

I'm trying to hunt down where this is called, anyone found it before?