I've noticed that every time I edit a node I get another blank duration field listed below.
From what I can tell there is no way to delete the fields even though they are blank.
I also have comment_cck enabled and comments showing up on the same page so perhaps that is what is causing it to get incremented everytime I edit a node.

Either way it seems to me that there should be someway to remove a duration and also that they should be growing like this.