Avoid the duplicated entries of GitHub releases

tool/gen-github-release.rb

@@ -27,6 +27,8 @@ client = Octokit::Client.new
 
 note =  "## What's Changed\n\n"
 
+notes = []
+
 diff = client.compare("ruby/ruby", ARGV[0], ARGV[1])
 diff[:commits].each do |c|
   if c[:commit][:message] =~ /\[(Backport|Feature|Bug) #(\d*)\]/
@@ -40,11 +42,15 @@ diff[:commits].each do |c|
   else
     next
   end
-  note << "* [#{title}](#{url})\n"
+  notes << "* [#{title}](#{url})"
 rescue OpenURI::HTTPError
   puts "Error: #{url}"
 end
 
+notes.uniq!
+
+note << notes.join("\n")
+
 note << "\n"
 note << "Note: This list is automatically generated by tool/gen-github-release.rb. Because of this, some commits may be missing.\n\n"
 note << "## Full Changelog\n\n"