How to get the overridden values from map ??

Suppose if i have Map object I am adding. object in map.

As we know map doesn't allow duplicate keys?

Map<Integer, String> map=new HashMap<>();

map.put(1, "Test");

map.put(2, "Testing");

map.put(1, "Duplicate key");

we know the key(1) is overridden with new value "Duplicate",

How do we find all keys which is overridden ???

And please can you Guys how java handles collision in Map??

Thanks in advance.