I’m sorry to hear your user is having trouble duplicating pages.
It looks like the plugin in question might be removing the backslash that Gutenberg adds when saving HTML (converted to unicode) to the database.
Could you possibly confirm this is not happening with another non-Cwicly Gutenberg page?
This is unfortunately unrelated to Cwicly, but more so how the plugins are duplicating the post/page.
When duplicating, these plugins will be retrieving post_content from the post and simply update the newly created post with the duplicate post content.
The issue here is that Gutenberg transforms to unicode all tags if not in the block HTML → we store our content in attributes for various reasons.
Basically, this requires a simple wp_slash() of the post content to properly process the changes Gutenberg makes.
I highly recommend reaching out to the respective developers as this will concern any Gutenberg plugin that uses attributes to save HTML content.