Implement URL conflict checking in `jekyll-doctor`.

lib/jekyll/commands/doctor.rb

@@ -6,23 +6,60 @@ module Jekyll
           site = Jekyll::Site.new(options)
           site.read
 
-          unless deprecated_relative_permalinks(site)
+          unless unhealthy(site)
             Jekyll.logger.info "Your test results", "are in. Everything looks fine."
           end
         end
 
+        def unhealthy(site)
+          [
+            deprecated_relative_permalinks(site),
+            conflicting_urls(site)
+          ].any?
+        end
+
         def deprecated_relative_permalinks(site)
           contains_deprecated_pages = false
           site.pages.each do |page|
             if page.uses_relative_permalinks
               Jekyll.logger.warn "Deprecation:", "'#{page.path}' uses relative" +
                                   " permalinks which will be deprecated in" +
-                                  " Jekyll v1.1 and beyond."
+                                  " Jekyll v1.2 and beyond."
               contains_deprecated_pages = true
             end
           end
           contains_deprecated_pages
         end
+
+        def conflicting_urls(site)
+          conflicting_urls = false
+          urls = {}
+          urls = collect_urls(urls, site.pages, site.dest)
+          urls = collect_urls(urls, site.posts, site.dest)
+          #urls = collect_urls(urls, site.static_files)
+          urls.each do |url, paths|
+            if paths.size > 1
+              conflicting_urls = true
+              Jekyll.logger.warn "Conflict:", "The URL '#{url}' is the destination" +
+                " for the following pages: #{paths.join(", ")}"
+            end
+          end
+          conflicting_urls
+        end
+
+        private
+
+        def collect_urls(urls, things, destination)
+          things.each do |thing|
+            dest = thing.destination(destination)
+            if urls[dest]
+              urls[dest] << thing.path
+            else
+              urls[dest] = [thing.path]
+            end
+          end
+          urls
+        end
       end
     end
   end