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

From javadocs for 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."

Thus the map value will be replaced, (which is a constant static field in HashSet class, and thus the same instance is replaced), and the map key is kept untouched (which, in fact IS the Set collection item)