diff --git a/benchmark/xpath/evaluator/concurrent_time_bench.rb b/benchmark/xpath/evaluator/concurrent_time_bench.rb
new file mode 100644
index 0000000..96a0bcf
--- /dev/null
+++ b/benchmark/xpath/evaluator/concurrent_time_bench.rb
@@ -0,0 +1,51 @@
+require_relative '../../benchmark_helper'
+require 'thread'
+
+puts 'Preparing...'
+
+xml       = read_big_kaf
+xpath     = 'KAF/terms/term'
+xpath_ast = Oga::XPath::Parser.new(xpath).parse
+output    = Queue.new
+
+stop    = false
+threads = []
+
+thread_count = ENV['THREADS'] ? ENV['THREADS'].to_i : 5
+
+trap 'INT' do
+  stop = true
+end
+
+# Parse these outside of the profiler
+documents = thread_count.times.map { Oga.parse_xml(xml) }
+
+puts 'Starting threads...'
+
+require 'profile' if ENV['PROFILE']
+
+thread_count.times.each do
+  threads << Thread.new do
+    oga_doc   = documents.pop
+    evaluator = Oga::XPath::Evaluator.new(oga_doc)
+
+    10.times do
+      break if stop
+
+      output << Benchmark.measure { evaluator.evaluate_ast(xpath_ast) }
+    end
+  end
+end
+
+threads.each(&:join)
+
+samples = []
+
+until output.empty?
+  samples << output.pop.real
+end
+
+average = samples.inject(:+) / samples.length
+
+puts "Samples: #{samples.length}"
+puts "Average: #{average.round(4)} seconds"
diff --git a/lib/oga/xml/node_set.rb b/lib/oga/xml/node_set.rb
index 78213ed..3ebcaa9 100644
--- a/lib/oga/xml/node_set.rb
+++ b/lib/oga/xml/node_set.rb
@@ -42,10 +42,15 @@ module Oga
       # @param [Oga::XML::NodeSet] owner The owner of the set.
       #
       def initialize(nodes = [], owner = nil)
-        @nodes = nodes
-        @owner = owner
+        @nodes    = nodes
+        @owner    = owner
+        @existing = {}
 
-        @nodes.each { |node| take_ownership(node) } if owner
+        @nodes.each do |node|
+          mark_existing(node)
+
+          take_ownership(node) if @owner
+        end
       end
 
       ##
@@ -103,11 +108,13 @@ module Oga
       # @param [Oga::XML::Node] node
       #
       def push(node)
-        return if @nodes.include?(node)
+        return if exists?(node)
 
         @nodes << node
 
-        take_ownership(node)
+        mark_existing(node)
+
+        take_ownership(node) if @owner
       end
 
       alias_method :<<, :push
@@ -118,11 +125,13 @@ module Oga
       # @param [Oga::XML::Node] node
       #
       def unshift(node)
-        return if @nodes.include?(node)
+        return if exists?(node)
 
         @nodes.unshift(node)
 
-        take_ownership(node)
+        mark_existing(node)
+
+        take_ownership(node) if @owner
       end
 
       ##
@@ -133,7 +142,11 @@ module Oga
       def shift
         node = @nodes.shift
 
-        remove_ownership(node)
+        if node
+          unmark_existing(node)
+
+          remove_ownership(node) if @owner
+        end
 
         node
       end
@@ -146,7 +159,11 @@ module Oga
       def pop
         node = @nodes.pop
 
-        remove_ownership(node)
+        if node
+          unmark_existing(node)
+
+          remove_ownership(node) if @owner
+        end
 
         node
       end
@@ -158,11 +175,13 @@ module Oga
       # @param [Oga::XML::Node] node
       #
       def insert(index, node)
-        return if @nodes.include?(node)
+        return if exists?(node)
 
         @nodes.insert(index, node)
 
-        take_ownership(node)
+        mark_existing(node)
+
+        take_ownership(node) if @owner
       end
 
       ##
@@ -257,7 +276,11 @@ module Oga
       def delete(node)
         removed = @nodes.delete(node)
 
-        remove_ownership(removed) if removed
+        if removed
+          unmark_existing(removed)
+
+          remove_ownership(removed) if @owner
+        end
 
         removed
       end
@@ -317,7 +340,7 @@ module Oga
       # @param [Oga::XML::Node] node
       #
       def take_ownership(node)
-        node.node_set = self if owner
+        node.node_set = self
       end
 
       ##
@@ -328,6 +351,22 @@ module Oga
       def remove_ownership(node)
         node.node_set = nil if node.node_set == self
       end
+
+      # @param [Oga::XML::Node|Oga::XML::Attribute] node
+      # @return [TrueClass|FalseClass]
+      def exists?(node)
+        @existing.key?(node)
+      end
+
+      # @param [Oga::XML::Node|Oga::XML::Attribute] node
+      def mark_existing(node)
+        @existing[node] = true
+      end
+
+      # @param [Oga::XML::Node|Oga::XML::Attribute] node
+      def unmark_existing(node)
+        @existing.delete(node)
+      end
     end # NodeSet
   end # XML
 end # Oga