Ok I found the cause. There is an option for each menu item (Layout tab) that is called Category. It has 4 options. If I choose the option that includes a link to the category, it triggers the creation of com_contacts links with the / at the end. That is because that link (on the category name) is a link to the category but on Joomla! Contatcs component. With just one menu item that include a link to a category on com_contacts, it propagates the creation of URLs to all linked categories. The issue is that those are duplicates in content. There is the one with the / at the end (com_contacts) and one without it (Trombinoscope) for each category

Thanks yes. I created a ticket with them but they told me the component just record the duplicates and I needed to find the cause of the issue.