P14 - Duplicate the elements of a list.
Author: Johan Viklund
Specification
P14 (*) Duplicate the elements of a list.
Example
> say ~dupli(<a b c c d>); a a b b c c c c d d
Source code: P14-viklund.pl
use v6; say <a b c c d>.map({ $_ xx 2 }).map({.list}).flat.list.perl;