Raise error when identifiers are not unique

lib/nanoc/base/errors.rb

@@ -221,6 +221,15 @@ def initialize(type, plural)
 
     end
 
+    # Error that is raised when multiple items or layouts with the same identifier exist.
+    class DuplicateIdentifier < Generic
+
+      def initialize(identifier, type)
+        super("There are multiple #{type}s with the #{identifier} identifier.")
+      end
+
+    end
+
   end
 
 end

lib/nanoc/base/source_data/site.rb

@@ -245,6 +245,10 @@ def load
       data_sources.each { |ds| ds.unuse }
       setup_child_parent_links
 
+      # Ensure unique
+      ensure_identifier_uniqueness(@items, 'item')
+      ensure_identifier_uniqueness(@layouts, 'layout')
+
       # Load compiler too
       # FIXME this should not be necessary
       compiler.load
@@ -346,6 +350,16 @@ def load_layouts
       end
     end
 
+    def ensure_identifier_uniqueness(objects, type)
+      seen = Set.new
+      objects.each do |obj|
+        if seen.include?(obj.identifier)
+          raise Nanoc::Errors::DuplicateIdentifier.new(obj.identifier, type)
+        end
+        seen << obj.identifier
+      end
+    end
+
     # Builds the configuration hash based on the given argument. Also see
     # {#initialize} for details.
     def build_config(dir_or_config_hash)

test/base/test_site.rb

@@ -113,6 +113,32 @@ def test_setup_child_parent_links
     end
   end
 
+  def test_multiple_items_with_same_identifier
+    with_site do
+      File.open('content/sam.html', 'w') { |io| io.write('I am Sam!') }
+      FileUtils.mkdir_p('content/sam')
+      File.open('content/sam/index.html', 'w') { |io| io.write('I am Sam, too!') }
+
+      assert_raises(Nanoc::Errors::DuplicateIdentifier) do
+        site = Nanoc::Site.new('.')
+        site.load
+      end
+    end
+  end
+
+  def test_multiple_layouts_with_same_identifier
+    with_site do
+      File.open('layouts/sam.html', 'w') { |io| io.write('I am Sam!') }
+      FileUtils.mkdir_p('layouts/sam')
+      File.open('layouts/sam/index.html', 'w') { |io| io.write('I am Sam, too!') }
+
+      assert_raises(Nanoc::Errors::DuplicateIdentifier) do
+        site = Nanoc::Site.new('.')
+        site.load
+      end
+    end
+  end
+
 end
 
 describe 'Nanoc::Site#initialize' do