Unique or non-unique indices

Here's an indexing question: I am in the process of adding a few indices to a given table; the table itself consists of over 3 million rows. I've had a tendency to just 'create index' without thought to the uniqueness of the index. If these indices are unique, is there any benefit to defining them as such? It appears that the optimizer doesn't really care if the index is unique or not. I did some testing and in some instances my cost actually increased, slightly. Did I miss something here? TIA John Carlson WH Smith, Inc.