Mirror part in an asembly or copy part in an asembly, Question.

I haven't used it yet, so I can't say how it works, but there is a new function in SW2015 that's supposed to do what you need.

If you don't have SW15...

2014 SOLIDWORKS Help - Mirroring a Part

Optionally, if you want to independently edit the features of the mirrored part without affecting the original part, under Link, click Break link to original part.You can also break the link to the original part later by listing the mirrored part's external references and selecting Break All. Once you break the link to the original, you cannot restore it.