Basicially when you go on the Beatmap listing on the osu! site, there should be like a feature to mark Beatmaps, like ingame the Collections, so you can mark the Beatmaps that you alreday have/had and don't want to play anymore.
If you are supporter you can press the osu!direct link on the website and it will move you to that map if you have it and play a preview in any case. That's also where you can choose to download it based on the preview.