About permanent link change

Hi Highend support,

I recently changed the permanent link to one of my product
from http://robofac.com/shop/hexapod/deep-invader/
to http://robofac.com/shop/hexapod/deep-invader-hexapod-kit/
but the old link still works and it will redirect you to the new link.
I’ve been building backlinks using both of them so if possible, I don’t want to lose any traffic. I google permanent link changes and folks are talking about using 301 redirect.
Just wondering if using 301 redirect is necessary or these two links were automatically merged together?

Thank you.