Duplicate Menu Item Titles are Ignored Even if They Belong on Different Menus

Problem/Motivation

If the migration contains duplicate menu item titles that belong to different menus ("menu_name"), they will still not be imported.

Steps to reproduce

1. Create a default_content file menu_link_content.menu_link_content.yml
2. Add at least two menu item entries with the same title. Both need their menu_name fields to be different.
3. Run the migration. I used drush migrate:import mdc_menu_link_content_menu_link_content.
4. The migration will indicate that it has completed the migration (all items should be imported). However, the count will not include the duplicates, as if they don't exist.
5. Looking at the menus on the site, you will see that those menu items are missing.

Proposed resolution

The ideal solution would simply be that if two duplicates are on different menus, that they would not be omitted.

Remaining tasks

User interface changes

API changes

Data model changes