Upgrade fails on 'index already exist'

Been trying to upgrade a rather large site to 7, but I keep running into this problem:

DatabaseSchemaObjectExistsException: Cannot add index system_list to table system: index already exists. in DatabaseSchema_mysql->addIndex() (line 424 of /drupal-7.0-alpha7/includes/database/mysql/schema.inc).

Not sure if its a bug or me doing something wrong.