Ensure attributes set on instance are calculated uniquely

This fixes a bug where assignment occurs multiple times; in the case of
nested attributes, assignment mutates the instance, meaning that it
occurring multiple times will cause issues.

Closes #314

lib/factory_girl/attribute_assigner.rb

@@ -37,7 +37,7 @@ module FactoryGirl
     end
 
     def attributes_to_set_on_instance
-      attribute_names_to_assign - @attribute_names_assigned
+      (attribute_names_to_assign - @attribute_names_assigned).uniq
     end
 
     def attributes_to_set_on_hash

spec/acceptance/nested_attributes_spec.rb

@@ -0,0 +1,32 @@
+require "spec_helper"
+
+describe "association assignment from nested attributes" do
+  before do
+    define_model("Post", :title => :string) do
+      has_many :comments
+      accepts_nested_attributes_for :comments
+    end
+
+    define_model("Comment", :post_id => :integer, :body => :text) do
+      belongs_to :post
+    end
+
+    FactoryGirl.define do
+      factory :post do
+        comments_attributes { [FactoryGirl.attributes_for(:comment), FactoryGirl.attributes_for(:comment)] }
+      end
+
+      factory :comment do
+        sequence(:body) {|n| "Body #{n}" }
+      end
+    end
+  end
+
+  it "assigns the correct amount of comments" do
+    FactoryGirl.create(:post).comments.count.should == 2
+  end
+
+  it "assigns the correct amount of comments when overridden" do
+    FactoryGirl.create(:post, :comments_attributes => [FactoryGirl.attributes_for(:comment)]).comments.count.should == 1
+  end
+end