DocumentFragment#dup avoids unnecessary copies.

[fixes #1063]

CHANGELOG.md

@@ -104,6 +104,7 @@ If you're offended by what happened here, I'd kindly ask that you comment on the
 * [MRI] OpenBSD installation should be a bit easier now. [#1685] (Thanks, @jeremyevans!)
 * [MRI] Cross-built Windows gems now support Ruby 2.5
 * Node#dup supports copying a node directly to a new document. See the method documentation for details.
+* [MRI] DocumentFragment#dup is now more memory-efficient, avoiding making unnecessary copies. [#1063]
 
 
 ## Bug fixes

lib/nokogiri/xml/document_fragment.rb

@@ -25,6 +25,15 @@ def initialize document, tags = nil, ctx = nil
         children.each { |child| child.parent = self }
       end
 
+      def dup
+        new_document = document.dup
+        new_fragment = XML::DocumentFragment.new(new_document)
+        children.each do |child|
+          child.dup(1, new_document).parent = new_fragment
+        end
+        new_fragment
+      end
+
       ###
       # return the name for DocumentFragment
       def name

test/xml/test_document_fragment.rb

@@ -263,6 +263,20 @@ def test_issue_1077_parsing_of_frozen_strings
         Nokogiri::XML::DocumentFragment.parse(input) # assert_nothing_raised
       end
 
+      def test_dup_creates_tree_with_identical_structure
+        original = Nokogiri::XML::DocumentFragment.parse("<div><p>hello</p></div>")
+        duplicate = original.dup
+        assert_equal original.to_html, duplicate.to_html
+      end
+
+      def test_dup_creates_mutable_tree
+        original = Nokogiri::XML::DocumentFragment.parse("<div><p>hello</p></div>")
+        duplicate = original.dup
+        duplicate.at_css("div").add_child("<b>hello there</b>")
+        assert_nil original.at_css("b")
+        assert_not_nil duplicate.at_css("b")
+      end
+
       if Nokogiri.uses_libxml?
         def test_for_libxml_in_context_fragment_parsing_bug_workaround
           10.times do