Fix: Not yet translated files in duplicated navigation entry

[AngryMane]

This PR fixes Not yet translated files in duplicated navigation entry.

root cause

For example, consider the following directory structure:

• docs
  • en
    • S3
      • test1.md
      • test2.md
  • den
    • S3
      • test2.md
      • test3.md

reconfigure_navigation removes the language info from the path, so the nav is modified as follows:

• docs
  • S3
    • test1.md
    • test2.md
  • S3
    • test2.md
    • test3.md

After this, there is no process to merge the same S3 directories, so nav_item.html in mkdocs-material renders each as a separate item, resulting in the problem you pointed out.

how to fix

if two sections meet all the following conditions , merge them.

• has same parent
• same title

[ultrabug]

Thanks a lot @AngryMane ; do you think you can make time to provide a test as well so we are sure to prevent any regression later on? If not, I will work on adding one when I can.