Ensure that each spec file is only loaded once.

- Fixes rspec#546, in which trying to run 2 or more specific examples in the
  same file caused the whole file to be run. This was happening because
  the following command caused the file to be loaded twice, and the filter
  was not being applied to the second instance:

    rspec path/to/file:37 path/to/file:42

lib/rspec/core/configuration.rb

@@ -695,7 +695,7 @@ def configure_expectation_framework
 
       # @private
       def load_spec_files
-        files_to_run.map {|f| load File.expand_path(f) }
+        files_to_run.uniq.map {|f| load File.expand_path(f) }
         raise_if_rspec_1_is_loaded
       end
 

spec/rspec/core/configuration_spec.rb

@@ -18,6 +18,12 @@ module RSpec::Core
         config.load_spec_files
       end
 
+      it "loads each file once, even if duplicated in list" do
+        config.files_to_run = ["a_spec.rb", "a_spec.rb"]
+        config.should_receive(:load).once
+        config.load_spec_files
+      end
+
       context "with rspec-1 loaded" do
         before do
           Object.const_set(:Spec, Module.new)

spec/rspec/core_spec.rb

@@ -3,29 +3,23 @@
 describe RSpec::Core do
 
   describe "#configuration" do
-
     it "returns the same object every time" do
       RSpec.configuration.should equal(RSpec.configuration)
     end
-
   end
 
   describe "#configure" do
-
     it "yields the current configuration" do
       RSpec.configure do |config|
         config.should eq(RSpec::configuration)
       end
     end
-
   end
 
   describe "#world" do
-
     it "returns the RSpec::Core::World instance the current run is using" do
       RSpec.world.should be_instance_of(RSpec::Core::World)
     end
-
   end
 
 end