P14 - Duplicate the elements of a list.

> p14:duplicate([1,2,3,4]).   
    [1,1,2,2,3,3,4,4]

erlang

%Duplicate the elements of a list.
-module(p14).
-export([duplicate/1]).

duplicate(Ls) ->
    lists:flatmap(fun(X) -> [X, X] end, Ls).