http://musicbrainz.org/doc/How_to_Use_the_Relationship_Editor#Batch-adding_relationships claims: "if there already existed vocal credits to Engelbert Humperdinck on tracks 1, 3, 5, and 9, and we used the batch-relationship dialog to add lead vocal credits, you would not end up with two vocal credits on these tracks. The relationship editor will modify the existing vocal credits by adding the "lead" attribute onto them"

In reality, we totally do end up with two vocal credits on these tracks. I'm not sure whether the docs are wrong or the relationship editor is (I can't remember it's ever done the batch-editing thing) but that'd be a very handy feature to have...