Fix duplicate contributor names when generating changelog

Follow up rubocop/rubocop@2d4b944

spec/tasks/changelog_spec.rb

@@ -41,10 +41,14 @@
     CHANGELOG
   end
 
+  let(:duplicate_entry) do
+    Changelog::Entry.new(type: :fix, body: 'Duplicate contributor name entry', user: 'johndoe')
+  end
+
   let(:entries) do
     %i[fix new fix].map.with_index do |type, i|
       Changelog::Entry.new(type: type, body: "Do something cool#{'x' * i}", user: "johndoe#{'x' * i}")
-    end
+    end << duplicate_entry
   end
   let(:entry) { entries.first }
 
@@ -72,6 +76,7 @@
 
       * [#x](https://github.com/rubocop/rubocop-ast/pull/x): Do something cool. ([@johndoe][])
       * [#x](https://github.com/rubocop/rubocop-ast/pull/x): Do something coolxx. ([@johndoexx][])
+      * [#x](https://github.com/rubocop/rubocop-ast/pull/x): Duplicate contributor name entry. ([@johndoe][])
     CHANGELOG
 
     expect(changelog.new_contributor_lines).to eq(

tasks/changelog.rb

@@ -134,9 +134,9 @@ def merge_content
   end
 
   def new_contributor_lines
-    contributors
-      .map { |user| format(CONTRIBUTOR, user: user) }
-      .reject { |line| @rest.include?(line) }
+    unique_contributor_names = contributors.map { |user| format(CONTRIBUTOR, user: user) }.uniq
+
+    unique_contributor_names.reject { |line| @rest.include?(line) }
   end
 
   def contributors