Collection interface just specifies groups of objects known as elements. Each concrete implementation of a Collection can choose its own way of how to maintain and order its elements.

Some collections may allow duplicate keys, while other collections may not.

A lot of collection implementations have clone method. But many do not. It is not worthwhile to include it in all, since Collection is an abstract representation. What matters is the concrete implementation.

Cloning and serialization come into picture while doing concrete implementation. Therefore, the concrete implementations of collections should decide how they can be cloned or serialized.