After losing or winning and unlock a plant trait, if you meet a plant with that same trait in later rounds, you can see that trait with the magnifying glass tool. Just click the magnifying glass, hover over the plant, and see the popup.
That's also where to know if it's a plant that have never seen before. If the traits are locked, then yes. If you can see the trait, then it's someong you met before.
Thank you so much for your feedback! Guess we should really add a lot more functions to that magnifying glass, hints, tips, soil status and all.