filter :groups_id, :as => :check_boxes, :collection => proc {Group.all}

class MyModel
  has_and_belongs_to_many :things, :uniq => true
end

distinct: true

controller do
  def apply_filtering(chain)
     @search = chain.ransack clean_search_params params[:q]
     @search.result(distinct: true)
   end
end

def apply_filtering(chain)
  super
  @search.result(distinct: true)
end