In external memory, is grouping equal elements easier than sorting?
Even testing whether $n$ elements are distinct is known to require $\Omega(n \log n)$ time on a model with some pretty reasonable restrictions.  See, for example, 
Anna Lubiw, András Rácz:
A Lower Bound for the Integer Element Distinctness Problem. Inf. Comput. 94(1): 83-92 (1991).