Siimetric Design o problemas con el diseño simétrico

Hello, I have a little problem, i beginning in simetric design, but I don’t make merge or union with vertices i put video in spanish:(
[video]http://youtu.be/TLMS0cB86yE[/video]
thanks in advance

Guess i saw you using Alt+d. Try Shift+d.
Still not sure i get your question right
.