It's very easy. Simply do what most people do: when someone tells or gives you something or sends you a link or a photo or anything you already have or have read or seen, first thing say, "Oh, I saw / knew / have that."
Me, I see a lot but you'll never, ever hear me saying that. Why? Because the instant you deflate the other person's bubble of joy at giving you something they think you'll like, you make it much more likely that next time they'll reflexively think, "the heck with him/her, why bother?" And you'll be sure to miss out on something that would've indeed been a wonderful surprise as a result of your having been so full of yourself you just had to show how up to the minute you were.
Common courtesy demands graciousness, perhaps an increasingly lost art but still one worth cultivating, not only for your own good but that of those around you. A simple "thank you" works beautifully. Always has, always will.
ah, but, you probably already knew that.