Fix false positive conflicts for static files in a collection

lib/jekyll/site.rb

@@ -360,9 +360,13 @@ def documents
     end
 
     def each_site_file
+      seen_files = []
       %w(pages static_files_to_write docs_to_write).each do |type|
         send(type).each do |item|
+          next if seen_files.include?(item)
+
           yield item
+          seen_files << item
         end
       end
     end

test/test_site.rb

@@ -749,5 +749,14 @@ def convert(*_args)
 
       assert_includes site.static_files.map(&:relative_path), "_methods/extensionless_static_file"
     end
+
+    should "not be revisited in `Site#each_site_file`" do
+      site = fixture_site("collections" => { "methods" => { "output" => true } })
+      site.read
+
+      visited_files = []
+      site.each_site_file { |file| visited_files << file }
+      assert_equal visited_files.count, visited_files.uniq.count
+    end
   end
 end