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?