If you've seen a meme/video before that your friend just shared with you (because they wanted you to see it), and you tell them, "I've seen this already," instead of giving them a nice lol or 😂emoji, then you are a selfish friend.

Let them enjoy making you happy. Why ruin that for them? It's OK that you've seen the meme before, but you don't have to declare it. It accomplishes nothing besides making your friend feel less happy and probably decide to stop sending you so many memes. Terrible outcome.