diff --git a/CHANGELOG.md b/CHANGELOG.md
index e794a512c..5cf9ae712 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,6 @@
 # Main (unreleased)
 
+* Fix double installation of bundler on CI runs when no test script is specified (https://github.com/heroku/heroku-buildpack-ruby/pull/1073)
 * Bundler 2.x is now 2.1.4 (https://github.com/heroku/heroku-buildpack-ruby/pull/1052)
 * Persistent bundler config is now being set using the `BUNDLE_*` env vars (https://github.com/heroku/heroku-buildpack-ruby/pull/1039)
 * Rake task "assets:clean" will not get called if it does not exist (https://github.com/heroku/heroku-buildpack-ruby/pull/1018)
diff --git a/Gemfile.lock b/Gemfile.lock
index 8480cd130..133bca93c 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -11,7 +11,7 @@ GEM
       excon
       moneta
       multi_json (>= 1.9.2)
-    heroku_hatchet (7.1.3)
+    heroku_hatchet (7.3.0)
       excon (~> 0)
       platform-api (~> 3)
       rrrretry (~> 1)
diff --git a/bin/support/ruby_test b/bin/support/ruby_test
index 977e9b8e1..6ecdab8f5 100755
--- a/bin/support/ruby_test
+++ b/bin/support/ruby_test
@@ -29,26 +29,36 @@ def execute_command(command)
   pipe(command, :user_env => true, :output_object => Kernel)
 end
 
+def user_env_hash
+  LanguagePack::ShellHelpers.user_env_hash
+end
+
 # $ bin/test BUILD_DIR ENV_DIR ARTIFACT_DIR
 build_dir, env_dir, _ = ARGV
 LanguagePack::ShellHelpers.initialize_env(env_dir)
+Dir.chdir(build_dir)
+
+# The `ruby_test-compile` program installs a version of Ruby for the
+# user's application. It needs the propper `PATH`, where ever Ruby is installed
+# otherwise we end up using the buildpack's version of Ruby
+#
+# This is needed here because LanguagePack::Ruby.slug_vendor_base shells out to the user's ruby binary
+user_env_hash["PATH"] = "#{build_dir}/bin:#{ENV["PATH"]}"
 
 bundler = LanguagePack::Helpers::BundlerWrapper.new(
-  gemfile_path: "#{build_dir}/Gemfile"
+  gemfile_path: "#{build_dir}/Gemfile",
+  bundler_path: LanguagePack::Ruby.slug_vendor_base # This was previously installed by bin/support/ruby_test-compile
 )
 
-# The `ruby_test-compile` program installs a version of Ruby for the
-# user's application. It needs the propper `PATH`, where ever Ruby is installed
-# we always add a symlink to the `bin/ruby` file so that is always valid.
-# We calculate the gem path the same way we do when compiling.
-LanguagePack::ShellHelpers.user_env_hash["PATH"] = "#{build_dir}/bin:#{bundler.bundler_path}/bin:#{ENV["PATH"]}"
-LanguagePack::ShellHelpers.user_env_hash["GEM_PATH"] = LanguagePack::Ruby.slug_vendor_base
+# - Add bundler's bin directory to the PATH
+# - Always make sure `$HOME/bin` is first on the path
+user_env_hash["PATH"] = "#{build_dir}/bin:#{bundler.bundler_path}/bin:#{user_env_hash["PATH"]}"
+user_env_hash["GEM_PATH"] = LanguagePack::Ruby.slug_vendor_base
 
-# load bundler
+# - Sets BUNDLE_GEMFILE
+# - Loads bundler's internal Gemfile.lock parser so we can use `bundler.has_gem?`
 bundler.install
 
-Dir.chdir(build_dir)
-
 execute_test(
   if bundler.has_gem?("rspec-core")
     if File.exist?("bin/rspec")
diff --git a/lib/language_pack/helpers/bundler_wrapper.rb b/lib/language_pack/helpers/bundler_wrapper.rb
index 9ca42c9ba..364f7c2c9 100644
--- a/lib/language_pack/helpers/bundler_wrapper.rb
+++ b/lib/language_pack/helpers/bundler_wrapper.rb
@@ -176,6 +176,14 @@ def fetch_bundler
       topic("Installing bundler #{@version}")
       bundler_version_escape_valve!
 
+      # Install directory structure (as of Bundler 2.1.4):
+      # - cache
+      # - bin
+      # - gems
+      # - specifications
+      # - build_info
+      # - extensions
+      # - doc
       FileUtils.mkdir_p(bundler_path)
       Dir.chdir(bundler_path) do
         @fetcher.fetch_untar(@bundler_tar)
diff --git a/lib/language_pack/ruby.rb b/lib/language_pack/ruby.rb
index 0583c0a0d..7551c970b 100644
--- a/lib/language_pack/ruby.rb
+++ b/lib/language_pack/ruby.rb
@@ -231,13 +231,14 @@ def warn_bundler_upgrade
     end
   end
 
-
   # For example "vendor/bundle/ruby/2.6.0"
   def self.slug_vendor_base
-    command = %q(ruby -e "require 'rbconfig';puts \"vendor/bundle/#{RUBY_ENGINE}/#{RbConfig::CONFIG['ruby_version']}\"")
-    slug_vendor_base = run_no_pipe(command, user_env: true).chomp
-    error "Problem detecting bundler vendor directory: #{@slug_vendor_base}" unless $?.success?
-    return slug_vendor_base
+    @slug_vendor_base ||= begin
+      command = %q(ruby -e "require 'rbconfig';puts \"vendor/bundle/#{RUBY_ENGINE}/#{RbConfig::CONFIG['ruby_version']}\"")
+      out = run_no_pipe(command, user_env: true).chomp
+      error "Problem detecting bundler vendor directory: #{out}" unless $?.success?
+      out
+    end
   end
 
   # the relative path to the bundler directory of gems
diff --git a/lib/language_pack/shell_helpers.rb b/lib/language_pack/shell_helpers.rb
index a7c4f2c09..00fe81dac 100644
--- a/lib/language_pack/shell_helpers.rb
+++ b/lib/language_pack/shell_helpers.rb
@@ -243,7 +243,7 @@ def topic(message)
     # (indented by 6 spaces)
     # @param [String] message to be displayed
     def puts(message)
-      message.each_line do |line|
+      message.to_s.each_line do |line|
         if line.end_with?("\n".freeze)
           print "       #{line}"
         else
diff --git a/spec/hatchet/ci_spec.rb b/spec/hatchet/ci_spec.rb
index 775e7fb40..473ab270b 100644
--- a/spec/hatchet/ci_spec.rb
+++ b/spec/hatchet/ci_spec.rb
@@ -4,6 +4,9 @@
   it "Does not cause the double ruby rainbow bug" do
     Hatchet::Runner.new("heroku-ci-json-example").run_ci do |test_run|
       expect(test_run.status).to eq(:succeeded)
+
+      install_bundler_count = test_run.output.scan("Installing bundler").count
+      expect(install_bundler_count).to eq(1), "Expected output to only install bundler once but was found #{install_bundler_count} times. output:\n#{test_run.output}"
     end
   end
 
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index 22688d3ac..21007b31b 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -7,6 +7,8 @@
 require 'language_pack'
 require 'language_pack/shell_helpers'
 
+ENV["HATCHET_BUILDPACK_BASE"] = "https://github.com/heroku/heroku-buildpack-ruby"
+
 ENV['RACK_ENV'] = 'test'
 
 DEFAULT_STACK = 'heroku-18'