Not sure whether it will do what you want, but look at http://docs.daz3d.com/doku.php/artzone/pub/software/symmetry/start
Alternatiely copy the item as suggested above and change the horizontal scale (probably the X-scale) to -100% as suggested by cunningf
I am assuming DS rather than Poser....

Malwat

Getting younger by the day; getting older by the minute....