Working on some code I wish to distribute via gem.  When I up the
version and do a gem update my-code, gem reports that both the
previous version and the new version are installed.  This doesn't seem
to occur with any other gems that I have.  I cannot seem to figure out
what's up with this.

Here's my gemspec:

spec = Gem::Specification.new do |s|
  s.name = "My Proggy"
  s.version = "0.5.3"
  s.author = "David Copeland"
  s.email = "my / email.com"
  s.homepage = "http://www.mystuff.com"
  s.platform = Gem::Platform::RUBY
  s.summary = "Some description"
  s.files = FileList["{bin,lib}/**/*"].to_a
  s.require_path = "lib"
  s.test_files = FileList["{test}/**/test*.rb"].to_a
  s.bindir = "bin"
  s.executables << "cl"
  s.has_rdoc = true
  s.rdoc_options << '--title' << 'Titlet' << '--main' << 'README.rdoc'
  s.extra_rdoc_files = ["README.rdoc"]
end

When I changed from "0.5.3" to "0.5.4", gem just installed it
alongside the old version.  Is there a way to tell it not to do that?

This happened installing the gem from a local .gem file AND from a
remote repository.