The general advice I have read on this issue is that you should avoid repeating yourself if possible, but its okay to go more in depth into a certain topic if it really fits with the prompt. Obviously don't copy and paste.
You have the freedom to write whatever you want in the secondary. But pay attention to the prompt. For example, if the prompt asks you for "a specific experience in which you promoted diversity," talk about one specific experience. If the prompt asks "how would you bring diversity to the class," you can talk about more examples than just one. Try to put a different spin on things you've already talked about in a PS or in the work/activities section.