Figured I'd better put this here, instead of in a comment.

When you share a link that someone else has already added, your tags will overwrite the other person's tags. Which is hugely annoying but there's nothing we can do about it. So please keep on adding!

I really hope the developers of Diigo will include something so links don't get shared twice, and labels don't get overwritten. You can vote for the feature here: http://feedback.diigo.com/forums/76211-ideas/suggestions/2293735-keep-existing-tags-when-resharing-a-document-to-a-

Fingers crossed!