We see this statement very often. When Pinterest got popular, how long until we saw replicas? When Medium and Svbtle were introduced, wasn't it just a matter of hours to me-too replicas came along? There was one fellow who's name escapes me that came up with the hidden-left interface menu that you swipe into view from the top that's now commonly used in the Facebook app and many, many mobile sites.
None of this is new, and there's nothing wrong with it. Copying is the biggest form of flattery as is often said, and its a validation of your idea.
What happens very often is that we see great ideas in their finished, polished form. Someone's showed us the path.
When you show me how to build a bridge, I don't have to invent a bridge. In many cases, I see your awesome bridge and figure out even better ways to build it. But coming up with that first bridge, doing the engineering and testing and iterations on it, that's hard.
That's really hard.
So yeah, you can recreate what someone else made in a lot shorter time than it took them to invent it. But invent something new and you'll see it takes you a long time too.