Endnotes in ePub use identical id's where an id should be Unique
When InDesign 18.0 and earlier versions make a reflowable ePub, the endnotes cause the file to be rejected when checked by https://www.ebookit.com/tools/bp/Bo/eBookIt/epub-validator. The reason: each endnote reference is embedded in a span with an id twice, and the name or identifier of that id is the same. In HTML (which is the construct of an ePub) each id should be unique. You cannot have a HTML file with two elements with the same id-name. The next line can be found at the endnote reference, but a similar thing coccurs on the backlink, back from the endnote text to the reference.
<span id="endnote-002-backlink"><a href="Naamloos-1.xhtml#endnote-002"><span id="endnote-002-backlink">2</span></a></span>
So in order to end up with an ePub that gets approved, I need to remove or rename one of the spans. This is something that needs to be fixed. I can't imagine why this error hasn't been mentioned earlier. The links back and forth do work, but this HTML/ePub is really not what it is supposed to be. For each endnote I have to make two corrections.
Fix for this issue is now available in the latest release of InDesign 2023 - that is, InDesign 18.1 version.
Please update to this version of InDesign to get the fix.
Adobe InDesign team