After upgrading to Rails 6.1.7.2 one of our apps printed a wall of warnings while booting:

/var/www/app/shared/bundle/ruby/2.6.0/gems/net-protocol-0.2.1/lib/net/protocol.rb:68: warning: already initialized constant Net::ProtocRetryError
/home/deploy-app/.rbenv/versions/2.6.10/lib/ruby/2.6.0/net/protocol.rb:66: warning: previous definition of ProtocRetryError was here
/var/www/app/shared/bundle/ruby/2.6.0/gems/net-protocol-0.2.1/lib/net/protocol.rb:214: warning: already initialized constant Net::BufferedIO::BUFSIZE
/home/deploy-app/.rbenv/versions/2.6.10/lib/ruby/2.6.0/net/protocol.rb:206: warning: previous definition of BUFSIZE was here
/var/www/app/shared/bundle/ruby/2.6.0/gems/net-protocol-0.2.1/lib/net/protocol.rb:541: warning: already initialized constant Net::NetPrivate::Socket
/home/deploy-app/.rbenv/versions/2.6.10/lib/ruby/2.6.0/net/protocol.rb:503: warning: previous definition of Socket was here

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We could fix this by adding the following to our Gemfile:

gem 'net-http', require: false
gem 'net-imap', require: false
gem 'net-protocol', require: false
gem 'net-smtp', require: false

Background

• https://github.com/ruby/net-imap/issues/16 Show archive.org snapshot
• https://github.com/shakacode/shakapacker/issues/206 Show archive.org snapshot

Alternative

Turned out that it was caused by an update of the mail gem from 2.7.0 to 2.8.0.1.
So another possibility would be to stay at version 2.7.0.