dups.erl

-module(dups).

-export([
         duplicates/1,
         same_order_duplicates/1
        ]).

duplicates([]) -> {[], []};
duplicates(List) ->
    [H|T] = lists:sort(List),
     duplicates_(H, T, [], [H]).
duplicates_(_, [], DupAcc, Acc) ->
    {DupAcc, Acc};
duplicates_(H, [H|T], [H|_]=DupAcc, Acc) ->
    duplicates_(H, T, DupAcc, Acc);
duplicates_(H, [H|T], DupAcc, Acc) ->
    duplicates_(H, T, [H|DupAcc], Acc);
duplicates_(_, [H|T], DupAcc, Acc) ->
    duplicates_(H, T, DupAcc, [H|Acc]).

same_order_duplicates([]) -> {[], []};
same_order_duplicates(List) ->
    List2 = lists:zip(List, lists:seq(1, length(List))),
    [{H,_}=H0|T] = lists:sort(List2),
    same_order_duplicates_(H, T, [], [H0]).
same_order_duplicates_(_, [], DupAcc, Acc) ->
    Acc2 = lists:sort(fun({_, A}, {_, B}) -> A<B end, Acc),
    Acc3 = [V||{V, _} <- Acc2],
    {DupAcc, Acc3};
same_order_duplicates_(H, [{H,_}|T], [H|_]=DupAcc, Acc) ->
    same_order_duplicates_(H, T, DupAcc, Acc);
same_order_duplicates_(H, [{H,_}|T], DupAcc, Acc) ->
    same_order_duplicates_(H, T, [H|DupAcc], Acc);
same_order_duplicates_(_, [{H,_}=H0|T], DupAcc, Acc) ->
    same_order_duplicates_(H, T, DupAcc, [H0|Acc]).