Raise on multiple invocations of javascript_pack_tag and stylesheet_pack_tag helpers

Gemfile.lock

@@ -1,7 +1,7 @@
 PATH
   remote: .
   specs:
-    shakapacker (6.0.0.rc.6)
+    shakapacker (6.0.0.rc.13)
       activesupport (>= 5.2)
       rack-proxy (>= 0.6.1)
       railties (>= 5.2)

lib/webpacker/helper.rb

@@ -96,6 +96,13 @@ def favicon_pack_tag(name, **options)
   #   <%= javascript_pack_tag 'calendar' %>
   #   <%= javascript_pack_tag 'map' %>
   def javascript_pack_tag(*names, defer: true, **options)
+    if @javascript_pack_tag_loaded
+      raise "To prevent duplicated chunks on the page, you should call javascript_pack_tag only once on the page. " \
+      "Please refer to https://github.com/shakacode/shakapacker/blob/master/README.md#usage for the usage guide"
+    end
+
+    @javascript_pack_tag_loaded = true
+
     javascript_include_tag(*sources_from_manifest_entrypoints(names, type: :javascript), **options.tap { |o| o[:defer] = defer })
   end
 
@@ -141,6 +148,13 @@ def preload_pack_asset(name, **options)
   #   <%= stylesheet_pack_tag 'calendar' %>
   #   <%= stylesheet_pack_tag 'map' %>
   def stylesheet_pack_tag(*names, **options)
+    if @stylesheet_pack_tag_loaded
+      raise "To prevent duplicated chunks on the page, you should call stylesheet_pack_tag only once on the page. " \
+      "Please refer to https://github.com/shakacode/shakapacker/blob/master/README.md#usage for the usage guide"
+    end
+
+    @stylesheet_pack_tag_loaded = true
+
     return "" if Webpacker.inlining_css?
 
     stylesheet_link_tag(*sources_from_manifest_entrypoints(names, type: :stylesheet), **options)

test/helper_test.rb

@@ -12,6 +12,9 @@ def base_url
         "https://example.com"
       end
     end.new
+
+    @javascript_pack_tag_loaded = nil
+    @stylesheet_pack_tag_loaded = nil
   end
 
   def test_asset_pack_path
@@ -129,6 +132,18 @@ def test_javascript_pack_tag_symbol
       javascript_pack_tag(:application)
   end
 
+  def test_javascript_pack_tag_multiple_invocations
+    error = assert_raises do
+      javascript_pack_tag(:application)
+      javascript_pack_tag(:bootstrap)
+    end
+
+    assert_equal \
+      "To prevent duplicated chunks on the page, you should call javascript_pack_tag only once on the page. " +
+        "Please refer to https://github.com/shakacode/shakapacker/blob/master/README.md#usage for the usage guide",
+      error.message
+  end
+
   def application_stylesheet_chunks
     %w[/packs/1-c20632e7baf2c81200d3.chunk.css /packs/application-k344a6d59eef8632c9d1.chunk.css]
   end
@@ -156,4 +171,16 @@ def test_stylesheet_pack_tag_splat
       (application_stylesheet_chunks).map { |chunk| stylesheet_link_tag(chunk, media: "all") }.join("\n"),
       stylesheet_pack_tag("application", media: "all")
   end
+
+  def test_stylesheet_pack_tag_multiple_invocations
+    error = assert_raises do
+      stylesheet_pack_tag(:application)
+      stylesheet_pack_tag(:hello_stimulus)
+    end
+
+    assert_equal \
+      "To prevent duplicated chunks on the page, you should call stylesheet_pack_tag only once on the page. " +
+        "Please refer to https://github.com/shakacode/shakapacker/blob/master/README.md#usage for the usage guide",
+      error.message
+  end
 end