How does MongoDB constuct indexes for a field with duplicate values in collections or an array field

How does MongoDB construct indexes for a field with duplicate values in the collections?

There are different methods to point duplicate records in B+Tree. Therefore To be in precise, I would like to know

  1. Does every duplicate key also have key - document pointer at leaf node? or Does every unique key has single key value and list of associated document pointers f? to optimize storage space without compromising Index scan cost.

2)how does it manage multi-key indexes on Array field? That reduce index size of Array size ?