How does an element that already exists in TreeMap behave when trying to insert it again?

In TreeMap, when attempting to insert an element with a key that already exists, the old value for that key will be replaced with the new one. That is, keys are unique, and re-inserting with the same key updates the corresponding value.

Example:

TreeMap<Integer, String> map = new TreeMap<>();
map.put(1, "one");
map.put(1, "uno");
System.out.println(map.get(1));  // Will output "uno"

Thus, TreeMap does not store duplicate keys but updates the value upon re-insertion with the same key.