Steps to reproduce the issue

install from joomla-cms/staging @ f45c0da
Add a custom field, of type "editor"
Edit content item
Click the Read More TinyMCE Button
Click the Read More TinyMCE Button
Click the Read More TinyMCE Button
Click the Read More TinyMCE Button

Expected result

A read more <hr id="system-readmore" /> appears in the TinyMCE as a red dashed line in the editor on the Content type

Subsequently clicking the read more button should give a popup with

Actual result

Multiple read more dashed lines get added to the editor

Additional STEPS to replicate unexpected behaviour

So now you have clicked many times on read more in tinymce on the editor on the content tab, now go to the Fields tab, and in your custom field editor click read more in that editor

Note that ONE red dashed line is added (in the source code a <hr id="system-readmore" /> is added)

Note that Subseqently clicking this read more button correctly gives the popup warning above

System information (as much as possible)

Additional comments

PR #14520 can be improved to fix this