is_unique in Form Validation on UPDATE

We have a table that has a section called "slug".
Obviously slug needs to be unique and therefore when we insert we use "is_unique[article.slug]" which works perfectly fine.

However the problem is that when we want to edit that slug we need it to again check if is_unique BUT not compare it with itself.

Is there a right way around this, because at the moment when we submit the update and slug is not changed it considers it "used" and therefore passes an error.

Thanks.