The EE documentation (link: https://docs.expressionengine.com/latest/cp/publish/create.html ) states that, “URL Titles must be unique within a channel,” but the Edit / Publish Form validation in the backend prevents the submission of entries that have the same URL title even if they are in different channels (the system returns the “This URL title is being used by [existing entry]. URL titles must be unique.” error message).

This was a feature that worked in EE 2.x and still works in EE3.1.4, if the URL title is manipulated directly in the database. For example, if two entries are amended in the database to both have the URL title “test”, but one is in the “people” channel and one is in the “page” channel, the following template code is valid and renders correctly (with the two different entries displayed as expected):

{exp:channel:entries channel="people" url_title="test"}
 <h2>{title}</h2>
<p>{/exp:channel:entries}</p>

<p>{exp:channel:entries channel="page" url_title="test"}<br /></p><h2>{title}</h2>
<p>{/exp:channel:entries}

It seems like the Publish / Edit form validation needs amending. Either that or the documentation needs updating to add some clarity and reflect the fact that this functionality has been deprecated in EE3.