About the stream deduping method

[Brian Goetz]

> *About the term "duplicate".*  If it were always the case that the
> /first/ occurrence of 'obj' in a stream is the one preserved, and all
> the rest thereby deemed to be the "duplicates" and excluded, then there
> would be a stronger reason to like the use of "duplicates" here.  Since
> that's not the case, it's just "meh".

FWIW, we do make this promise for ordered streams (for unordered 
streams, by definition, you can't tell whether we adhere to this or not.)

> *If I had to choose. *I would probably just go with:   distinct().
> That's all SQL needed, and I think it did fine with it.

Works for me.