Grouping operator in DBMS (γ) is duplicate-impervious. Why?
In the book, "Garcia-Molina H., Ullman J., Widom J., Database Systems: The Complete Book. Pearson Prentice Hall, 2009", Chapter 5, "Algebraic and Logic Query Languages", on top of page 217 of the 2nd edition, there is a box titled "δ is a Special Case of γ", that explains how the duplicate removal operation δ is redundant, since it can be replaced by the use of γ. 

Simply group by all the attributes of the relation, without any aggregation. This will replace each group, consisting only of tuples wich coincide on all the attributes, with a single tuple. And this is exactly the semantics of the duplicate elimination operator.