Twitter embed in Classic Block is duplicated

[westonruter]

Bug Description

Given post_content (from theme unit test data) that consists of:

https://twitter.com/nacin/status/319508408669708289

This post tests WordPress' <a title="Twitter Embeds" href="https://en.support.wordpress.com/twitter/twitter-embeds/" target="_blank" rel="noopener noreferrer">Twitter Embeds</a> feature.

This is incorrectly resulting in the Tweet being duplicated:

The generated HTML even includes the duplication:

<p><amp-twitter width="600" height="480" layout="responsive" data-tweetid="319508408669708289"></amp-twitter></p>
<amp-twitter width="600" height="480" layout="responsive" data-tweetid="319508408669708289" class="twitter-tweet" data-width="550" data-dnt="true" placeholder=""><blockquote class="twitter-tweet" data-width="550" data-dnt="true" placeholder="">
<p lang="en" dir="ltr">Really cool to read through and find so much awesomeness added to WordPress 3.6 while I was gone. I should take three weeks off more often.</p>
<p>— Andrew Nacin (@nacin) <a href="https://twitter.com/nacin/status/319508408669708289?ref_src=twsrc%5Etfw">April 3, 2013</a></p></blockquote></amp-twitter>
<p></p>
<p>This post tests WordPress’ <a title="Twitter Embeds" href="https://en.support.wordpress.com/twitter/twitter-embeds/" target="_blank" rel="noopener noreferrer">Twitter Embeds</a> feature.</p>

Expected Behaviour

The non-AMP version does not have a duplicated embed, and this is what it should look like in AMP:

Steps to reproduce

1. Create a post and enter code editor mode, pasting the above post_content into it.
2. Publish and view the AMP version/
3. See duplicated tweet.

Additional context

The issue doesn't seem to occur in 1.4.1, so it seems to be some regression introduced in develop since the 1.4 branch.

---

Do not alter or remove anything below. The following sections will be managed by moderators only.

Acceptance criteria

Implementation brief

QA testing instructions

Demo

Changelog entry

[csossi]

Now not seeing any duplication,: