Fix diff bottom expand button appears twice

This is a quick fix by only append match line once
when calling diff_lines_for_serializer multiple time.

Also enable feature by default

app/serializers/diff_file_entity.rb

@@ -57,7 +57,7 @@ class DiffFileEntity < DiffFileBaseEntity
     diff_file.diff_lines_for_serializer
   end
 
-  expose :is_fully_expanded, if: -> (diff_file, _) { Feature.enabled?(:expand_diff_full_file) && diff_file.text? } do |diff_file|
+  expose :is_fully_expanded, if: -> (diff_file, _) { Feature.enabled?(:expand_diff_full_file, default_enabled: true) && diff_file.text? } do |diff_file|
     diff_file.fully_expanded?
   end
 

changelogs/unreleased/59147-duplicate-match-line.yml

+---
+title: Fix duplicated bottom match line on merge request parallel diff view
+merge_request: 26402
+author:
+type: fixed

lib/gitlab/diff/file.rb

@@ -158,7 +158,10 @@ module Gitlab
         new_blob || old_blob
       end
 
-      attr_writer :highlighted_diff_lines
+      def highlighted_diff_lines=(value)
+        clear_memoization(:diff_lines_for_serializer)
+        @highlighted_diff_lines = value
+      end
 
       # Array of Gitlab::Diff::Line objects
       def diff_lines
@@ -314,10 +317,11 @@ module Gitlab
       # This adds the bottom match line to the array if needed. It contains
       # the data to load more context lines.
       def diff_lines_for_serializer
+        strong_memoize(:diff_lines_for_serializer) do
           lines = highlighted_diff_lines
 
-        return if lines.empty?
-        return if blob.nil?
+          next if lines.empty?
+          next if blob.nil?
 
           last_line = lines.last
 
@@ -328,6 +332,7 @@ module Gitlab
 
           lines
         end
+      end
 
       def fully_expanded?
         return true if binary?

spec/lib/gitlab/diff/file_spec.rb

@@ -72,6 +72,13 @@ describe Gitlab::Diff::File do
       expect(diff_file.diff_lines_for_serializer.last.type).to eq('match')
     end
 
+    context 'when called multiple times' do
+      it 'only adds bottom match line once' do
+        expect(diff_file.diff_lines_for_serializer.size).to eq(31)
+        expect(diff_file.diff_lines_for_serializer.size).to eq(31)
+      end
+    end
+
     context 'when deleted' do
       let(:commit) { project.commit('d59c60028b053793cecfb4022de34602e1a9218e') }
       let(:diff_file) { commit.diffs.diff_file_with_old_path('files/js/commit.js.coffee') }