In any ordinary object relationships, one would never have duplicates because it simply makes no sense to have the same object twice on one end of a relationship. Some Java Collection objects enforce this rule and others don’t.
In designing these collection classes, we decided to omit checking for duplicates for two reasons. First, in any implementation which does not provide direct indexing by object identity, searching every element already in a collection to see if it matches a proposed new element is expensive overhead. Second, in most cases, the very nature of adding elements to a collection will not lead to duplicates, so incurring the overhead of checking for duplicates is wasted effort. Compare, for example, to populating a temp-table with the lines for a particular order based on the persisted data – would you feel the need to include logic to check to see whether an order line was already in the table or would you assume, rightly, that sequential reading of the persisted data will result in no duplicates.
We are electing to provide a Contains() operator on our collection classes which the parent can use to test for duplicates if a situation arises in which it may not be clear if a particular object is already in the collection.
With key-value sets there is some question about what one means by “no duplicates”. In the Java Map classes, no duplicate keys are allowed based on the idea that each key should uniquely identify an object, but there is no restriction on duplicate objects, i.e., the same object can be pointed to by different keys. This makes little sense in a relationship collection. Consequently, we have provided two types of key/value pair objects, one of which limits to unique keys and the other of which doesn’t. The former is like the Java Map class and the later provides for collections ordered by an attribute, but not uniquely identified by them. Depending on implementation, these would or would not support duplicate objects, but there seems little reason to allow duplicates if it is easy to prevent.