diff --git a/lib/generators/chromium/pdf/install/install_generator.rb b/lib/generators/chromium/pdf/install/install_generator.rb
index 53f72d2..5bcd378 100644
--- a/lib/generators/chromium/pdf/install/install_generator.rb
+++ b/lib/generators/chromium/pdf/install/install_generator.rb
@@ -29,7 +29,9 @@ def include_chrome_buildpack_in_app_json
         def add_buildpack_to_app_json
           in_root do
             update_json_file('app.json') do |data|
-              data['buildpacks'] << { 'url' => 'heroku-community/chrome-for-testing' }
+              if data['buildpacks'].none? { |entry| entry['url'] == 'heroku-community/chrome-for-testing' }
+                data['buildpacks'] << { 'url' => 'heroku-community/chrome-for-testing' }
+              end
             end
           end
         end
diff --git a/test/generators/install_generator_test.rb b/test/generators/install_generator_test.rb
index ff199d9..3415c09 100644
--- a/test/generators/install_generator_test.rb
+++ b/test/generators/install_generator_test.rb
@@ -9,6 +9,7 @@ class InstallGeneratorTest < Rails::Generators::TestCase
   destination File.expand_path('../../tmp', __dir__)
   setup do
     prepare_destination
+    @app_json_path = "#{destination_root}/app.json"
   end
 
   test 'pdf job template is created' do
@@ -21,9 +22,7 @@ class InstallGeneratorTest < Rails::Generators::TestCase
     assert_file 'config/initializers/good_job.rb'
   end
 
-  test 'app.json is created' do
-    @app_json_path = "#{destination_root}/app.json"
-
+  test 'app.json is correct' do
     File.write(@app_json_path, ActiveSupport::JSON.encode({}))
     run_generator
     assert_includes(File.read(@app_json_path), 'heroku-community/chrome-for-testing')
@@ -32,4 +31,15 @@ class InstallGeneratorTest < Rails::Generators::TestCase
     run_generator
     assert_includes(File.read(@app_json_path), 'heroku-community/chrome-for-testing')
   end
+
+  test 'does not duplicate the buildpack in app.json' do
+    File.write(@app_json_path,
+      ActiveSupport::JSON.encode({ buildpacks: [{ 'url' => 'heroku-community/chrome-for-testing' }] }))
+
+    run_generator
+    found = ActiveSupport::JSON.decode(File.read(@app_json_path))['buildpacks'].select do |entry|
+      entry['url'] == 'heroku-community/chrome-for-testing'
+    end
+    assert_equal(1, found.size)
+  end
 end