If it's refering to a Map interface. The keys are used to identify an object in a collection. Example impllementations are hashTable and Hashmap. They store key/value pairs, and the keys can not be duplicate.
Bosun (SCJP, SCWCD)
So much trouble in the world -- Bob Marley
You would use Map. Yes, seriously. If you have a duplicate key with a different value, how would you know which value to return for that key? You can't know. All you can do is return all the values for that key, which corresponds to a Map that maps the key to a Collection of values. The key => Collection map is once again unique. - Peter