[Installer] Remove duplicates form #installed_specs

lib/cocoapods/hooks/library_representation.rb

@@ -28,6 +28,8 @@ module Pod
       #
       def prefix_header_path
         library.prefix_header_path
+        UI.warn "LibraryRepresentation#prefix_header_path is deprecated. " \
+          "Use the specification `prefix_header_contents` attribute."
       end
       alias :prefix_header_filename :prefix_header_path
 

lib/cocoapods/installer.rb

@@ -248,7 +248,7 @@ module Pod
       pod_installer.install_docs = config.install_docs?
       pod_installer.install!
       @pod_installers << pod_installer
-      @installed_specs.concat(specs_by_platform.values.flatten)
+      @installed_specs.concat(specs_by_platform.values.flatten.uniq)
     end
 
     # Cleans the sources of the Pods if the config instructs to do so.

spec/unit/installer_spec.rb

@@ -174,7 +174,7 @@ module Pod
           spec = fixture_spec('banana-lib/BananaLib.podspec')
           library = Library.new(nil)
           library.specs = [spec]
-          @installer.stubs(:libraries).returns([library])
+          @installer.stubs(:libraries).returns([library, library])
           @installer.instance_variable_set(:@installed_specs, [])
           Installer::PodSourceInstaller.any_instance.stubs(:install!)
           @installer.send(:install_source_of_pod, 'BananaLib')