How do I include the main navigation block twice?
This functionality is present out of the box in Drupal 8. You can actually place the same block multiple times in different regions.

For adding a block multiple times:


Go to Admin -> Structure -> Block layout (/admin/structure/block).
Click on "Place block" button besides the region name.
Select any block you wish to add to that region.


Follow the same steps to add block to a different region.