How does one offset after duplicating keys?

I'm not entirely sure exactly what you mean but I think you have copied the legs so they are doing exactly the same thing and now you want to shift them in time, yeah?

If it's an in-place cycle, you can just grab the keys and shift them and then delete the stuff you don't need (the stuff before and after the actual cycle)

In it's a translating walk cycle, you need to make sure that when it loops, the feet and the torso use the "cycle with offset" curve modifier.  Everything else is just a normal "cycle modifier"

I think that's what you're wanting to know.