I've got twitter tools posting a tweet when a new blog post is saved. It creates a custom field 'aktt_notify_twitter' initially set to 'no'. So I write my post, and set it to 'yes'. Save the post, tweet gets sent, new field called 'aktt_tweeted' appears, with value 'yes'. All good.
But next time I save the post, another custom field gets created, also called 'aktt_notify_twitter', and also set to 'yes'. So I end up with two custom fields, both called the same name, and both holding the same value. That can't be right, can it?