This report is limited to com_contacts but it could also touch some other components in core.

Steps to reproduce the issue

Use latest 4.0-dev
Tested on a multilingual site.
Create a contact category tagged to each language.
Create a contact in each of these categories, tagged to the same language.

I have as menu item type "List Contacts in a category"
/index.php/en/contact-category-en-gb

The link on that page displaying the contact is
/index.php/en/contact-category-en-gb/mycontact // see below direct menu item. this url is a double with the one below (it should be /index.php/en/my-contact). Alias of that single contact is mycontact

// I do also have a direct menu item to that contact (alias of the menu item is my_contact)
When I use this last one (/index.php/en/my-contact), the category link above the contact gives
/index.php/en/my-contact?view=category&id=14 instead of the existing /index.php/en/contact-category-en-gb