collection_select unique value

I have a table named "posts" with an "author" attribute. At the posts index page, the user should be able to sort the posts through a drop down list that contains the author name.

My problem is, when I use form option helper "collection_select", it gives me all the authors that contained in the "field" - 3 Johns and 4 Chris -. How can I remove the duplicates from the drop down list?