Ruby-on-rails – How come Factory Girl isn’t sequencing unique attributes

My controller spec fails because Factory Girl seems to be creating non-unique Users even though I sequence the User attributes that need to be unique.

The Errors

  1) TopicsController POST #create when topic is invalid should render new
     Failure/Error: let(:invalid_topic) {Factory.build :invalid_topic}
     ActiveRecord::RecordInvalid:Validation failed: Email has already been taken, Username has already been taken

  2) TopicsController POST #create when topic is valid should redirect to show
     Failure/Error: let(:valid_topic) {Factory.build :topic}
     ActiveRecord::RecordInvalid:
       Validation failed: Email has already been taken, Username has already been taken

The Controller Spec (RSpec)

  describe "POST #create" do                          
    let(:valid_topic) {Factory.build :topic}
    let(:invalid_topic) {Factory.build :invalid_topic}

    context "when topic is invalid" do
      it "should render new" do
        post :create, :topic => invalid_topic
        response.should render_template(:new)
      end
    end
    context "when topic is valid" do
      it "should redirect to show" do
        post :create, :topic => valid_topic
        response.should redirect_to(topic_path(assigns(:topic)))
      end
    end
  end

The Factories

Factory.define :user do |f|
  f.sequence(:username) { |n| "foo#{n}"}
  f.password "password"
  f.password_confirmation { |u| u.password}
  f.sequence(:email) { |n| "foo#{n}@example.com"}
end

Factory.define :topic do |f|
  f.name "test topic"
  f.association :creator, :factory => :user
  f.forum_id 1
end

Why isn't Factory Girl sequencing the User attributes when I use Factory.create :topic?