環境情報
- macOS Big Sur 11.6
- Ruby 2.6.1
- Rails 5.2.6
起きたこと
OSのアップデートやrbenvの再インストールを行った後、
rails s や rails c を実行した時に下記エラーが発生するようになってしまった
（起きていた現象として、エラーが発生するだけで起動は行われる）
/path/to/.rbenv/versions/2.6.1/lib/ruby/gems/2.6.0/gems/bundler-2.2.28/lib/bundler/rubygems_integration.rb:12: warning: already initialized constant Bundler::RubygemsIntegration::EXT_LOCK
/path/to/.rbenv/versions/2.6.1/lib/ruby/2.6.0/bundler/rubygems_integration.rb:12: warning: previous definition of EXT_LOCK was here
/path/to/.rbenv/versions/2.6.1/lib/ruby/gems/2.6.0/gems/bundler-2.2.28/lib/bundler/version.rb:4: warning: already initialized constant Bundler::VERSION
/path/to/.rbenv/versions/2.6.1/lib/ruby/2.6.0/bundler/version.rb:10: warning: previous definition of VERSION was here
/path/to/.rbenv/versions/2.6.1/lib/ruby/gems/2.6.0/gems/bundler-2.2.28/lib/bundler/constants.rb:4: warning: already initialized constant Bundler::WINDOWS
/path/to/.rbenv/versions/2.6.1/lib/ruby/2.6.0/bundler/constants.rb:4: warning: previous definition of WINDOWS was here
/path/to/.rbenv/versions/2.6.1/lib/ruby/gems/2.6.0/gems/bundler-2.2.28/lib/bundler/constants.rb:5: warning: already initialized constant Bundler::FREEBSD
/path/to/.rbenv/versions/2.6.1/lib/ruby/2.6.0/bundler/constants.rb:5: warning: previous definition of FREEBSD was here
/path/to/.rbenv/versions/2.6.1/lib/ruby/gems/2.6.0/gems/bundler-2.2.28/lib/bundler/constants.rb:6: warning: already initialized constant Bundler::NULL
/path/to/.rbenv/versions/2.6.1/lib/ruby/2.6.0/bundler/constants.rb:6: warning: previous definition of NULL was here
/path/to/.rbenv/versions/2.6.1/lib/ruby/gems/2.6.0/gems/bundler-2.2.28/lib/bundler/current_ruby.rb:12: warning: already initialized constant Bundler::CurrentRuby::KNOWN_MINOR_VERSIONS
/path/to/.rbenv/versions/2.6.1/lib/ruby/2.6.0/bundler/current_ruby.rb:12: warning: previous definition of KNOWN_MINOR_VERSIONS was here
/path/to/.rbenv/versions/2.6.1/lib/ruby/gems/2.6.0/gems/bundler-2.2.28/lib/bundler/current_ruby.rb:26: warning: already initialized constant Bundler::CurrentRuby::KNOWN_MAJOR_VERSIONS
/path/to/.rbenv/versions/2.6.1/lib/ruby/2.6.0/bundler/current_ruby.rb:24: warning: previous definition of KNOWN_MAJOR_VERSIONS was here
/path/to/.rbenv/versions/2.6.1/lib/ruby/gems/2.6.0/gems/bundler-2.2.28/lib/bundler/current_ruby.rb:28: warning: already initialized constant Bundler::CurrentRuby::KNOWN_PLATFORMS
/path/to/.rbenv/versions/2.6.1/lib/ruby/2.6.0/bundler/current_ruby.rb:26: warning: previous definition of KNOWN_PLATFORMS was here
解決方法
gem update --system を実行する（gemコマンドをアップデートする）