I had the same problem but with a newer version
$ gem list bundler
*** LOCAL GEMS ***
bundler (default: 2.1.4, default: 2.1.2)
But after following indications on this post Two default versions of rake installed - how to delete one allowed me to deleted one.
You have to delete the .gemspec file corresponding to the default gem you want to delete.
So first, locate where those files are.
# I'm running RVM to manage my Ruby versions
~/.rvm/rubies/ruby-2.7.0/lib/ruby/gems/2.7.0/specifications/default/
-rw-r--r-- 1 myuser staff 10731 Dec 26 17:22 bundler-2.1.2.gemspec
-rw-r--r-- 1 myuser staff 15134 Jan 7 17:30 bundler-2.1.4.gemspec
Delete the one you don't need.
$ rm ~/.rvm/rubies/ruby-2.7.0/lib/ruby/gems/2.7.0/specifications/default/bundler-2.1.2.gemspec
Then install (or reinstall) the gem you want to set as default.
$ gem install bundler:2.1.4 --default
Successfully installed bundler-2.1.4 as a default gem
Finally you'll have installed only the version you wanted.
$ gem list bundler
*** LOCAL GEMS ***
bundler (default: 2.1.4)
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