finding items that occur more than once in a list

[John Machin]

On Mar 20, 9:14 am, sturlamolden <sturlamol... at yahoo.no> wrote:
> On 19 Mar, 22:48, John Machin <sjmac... at lexicon.net> wrote:
>
> > I'd use Raymond Hettinger's solution. It is as much O(N) as Paul's,
> > and is IMHO more readable than Paul's.
>
> Is a Python set implemented using a hash table?

What don't you understand about the comments in the first two
screenfuls of Objects/setobject.c?