http://stackoverflow.com/questions/6699351/forcing-activerecord-to-count-distinct-with-kaminari

thing.rb

# when I call Thing.count, it works and DISTINCT records are selected
# when I call Thing.scoped.count (or any other query), it doesn't work

class Thing < ActiveRecord::Base
  class << self
    def count_with_distinct
      count_without_distinct(:distinct => true)
    end
    alias_method_chain :count, :distinct
  end

  ...
end