Hi Gorka!
Thanks for getting in touch. Let me help you with that issue.
The thread you linked about duplicate content has already been fixed long ago, so while yours might look the same, it could be due to a different issue.
Could you please share with us your system information by following this guide > https://theeventscalendar.com/knowledgebase/sharing-sys-info/ ? We’ll see if there is anything strange that would indicate an error from there.
Just to confirm, when switching to a default template, does it still duplicate the content?
Please let me know about it.
Thanks,
Victor