I know the feeling. In that circumstance, since the person apparently wants to talk about that news, I try to find something to ask them about it that I don't already know the answer to. Which could even just be to ask them how they feel about that news (which can be in the form "I bet you're happy about that...", etc), since I don't usually know that. That way, they get to talk about what they want to talk about and I get to hear about things I don't already know. Whether to make it clear that I already knew the basics or not depends on the circumstance.