non-www redirect to www- how can this be done?

I’m using WP 3.8 multisite and MU domain mapping…trying to change one of my sites from non-www to www. Currently I configure my sites in domain mapping twice, one with non-www and www (this being primary). However, I’m concerned that google views this as duplicate content. Does anyone have solution to this. I have been reading post after post but no one seems to have a solution.