Has anyone had issues with installing additional RAM on their builds?

Here's the thing, I have a set of Corsair 12gb ram in my desktop but with me running ram intensive programs I would like a bit more. So I was thinking of buying the same 12gb set and installing it.

I've heard that this may cause problems with the chips not syncing or something to that tune.

The jist of it: Is it viable to upgrade my ram by buying a second set of 12gb, or should I buy a new set of 24gb?