hashset uniquness

Adds the specified element to this set if it is not already present. More formally, adds the specified element e to this set if this set contains no element e2 such that (e==null ? e2==null : e.equals(e2)). If this set already contains the element, the call leaves the set unchanged and returns false.

The above quote is of course from the API:
http://download.oracle.com/javase/6/docs/api/java/util/HashSet.html

This may be also interesting discussion though the answer to your question is mostly in the posts above:
http://www.coderanch.com/t/418129/java-programmer-SCJP/certification/HashSet-internal-working

CustomerVO cus= new CustomerVO();
cus.setId("1");
Set set = new Hashset();
set.add(cus);
...

Later on in my code, i try to add the same cus object again
set.add(cus),

what error will  be thrown?

thx

According to API above there will be no Exception but if objects are equal the method will
return false, and the set will be unchanged

So it remains to you to check if method .add(..) returned true or false