next/script onReady does not fire when rendering multiple instances with the same src

Link to the code that reproduces this issue

https://github.com/carlyle/next-script-onready-reproduction-app

To Reproduce

You can see it on a built and deployed app:

1. Go to https://next-script-onready-reproduction-app.vercel.app/single. Verify that Embed #1 renders without error and with ✅s by "Loaded?" and "Ready?", which is expected
2. Go to https://next-script-onready-reproduction-app.vercel.app/multiple. Embed 1 renders without error and with ✅s by "Loaded?" and "Ready?" but the following embeds do not render as expected

As well as in local development:

1. Start the application (next dev)
2. Go to http://localhost:3000/single. Verify that Embed 1 renders without error and with ✅s by "Loaded?" and "Ready?", which is expected
3. Go to http://localhost:3000/multiple. Embed 1 renders without error and with ✅s by "Loaded?" and "Ready?" but the following embeds do not render as expected

Current vs. Expected behavior

Current Behavior

The first instance of <Script> with a particular src:

• fires the onLoad callback as expected
• fires the onReady callback as expected

The second instance of <Script> with the same src:

• sometimes fires the onLoad callback, which is unexpected because it is intended for "run[ning] JavaScript code once after the script has finished loading"
• does not fire the onReady callback, which is unexpected because it is intended for "run[ning] JavaScript code after the script has finished loading and every time the component is mounted"

The third and later instances of <Script> with the same src:

• do not fire the onLoad callback, which is expected
• do not fire the onReady callback, which is unexpected because it is intended for "run[ning] JavaScript code after the script has finished loading and every time the component is mounted"

Expected Behavior

The first instance of <Script> with a particular src:

• fires the onLoad callback
• fires the onReady callback

The second and later instances of <Script> with the same src:

• do not fire the onLoad callback
• fire the onReady callback

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 23.3.0: Wed Dec 20 21:31:00 PST 2023; root:xnu-10002.81.5~7/RELEASE_ARM64_T6020
  Available memory (MB): 32768
  Available CPU cores: 12
Binaries:
  Node: 20.11.0
  npm: 10.2.4
  Yarn: N/A
  pnpm: N/A
Relevant Packages:
  next: 14.2.0-canary.22 // Latest available version is detected (14.2.0-canary.22).
  eslint-config-next: N/A
  react: 18.2.0
  react-dom: 18.2.0
  typescript: 5.1.3
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

App Router, Script optimization (next/script)

Which stage(s) are affected? (Select all that apply)

next dev (local), next start (local), Vercel (Deployed), Other (Deployed)

Additional context

The motivation behind having a <Script> render within each instance of <Embed>, as opposed to having a single <Script> somewhere else in the app, is:

• to keep all of the embed-related logic in one place
• to prevent the remote script from loading if we don't end up rendering an embed

An example use case would be embedding videos in news articles: we want to load the video player library whenever an article has any video embeds, but wouldn't want to load that library unnecessarily on articles without video embeds

Start with the linked reproduction app and compare the /single and /multiple routes in next dev or a deployed build. Then inspect the next/script implementation and its callback handling for repeated instances with the same src; done means later instances consistently fire onReady while only the first fires onLoad.