How does HashSet not allow duplicates?

Question

I was going through the add method of HashSet. It is mentioned that

If this set already contains the element, the call leaves t

Answer 1

See HashMap#put:

Associates the specified value with the specified key in this map. If the map previously contained a mapping for the key, the old value is replaced.

It replaces the key with the new value, this way, no duplicates will be in the HashSet.