Locutions

I'd like to know how a French speaker might express this, especially when nothing specific follows it, e.g.> One of these things is usually enough, but I've bought a second one, just in... more

Je souhaite être éclairé concernant l'usage approprié de ces locutions qui sont fort usitées me semble-t-il.

I ever heard from my French teacher that "du coup" is used only for spoken usage, but I'm not sure about it.