Fix duplicate contributor names when generating changelog

Follow up rubocop/rubocop@2d4b944

Author: koic

tasks/changelog.rb

@@ -129,9 +129,9 @@ 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