You will receive three Cecilias after following a specific dialogue branch if you meet her for the first time. Many times, NPCs often give you rewards for interacting with them for the first time, and some rewards like the Dark Iron sword can also be obtained. The Dark Iron sword is only a single 3-star sword in the game that cannot be refined, and you can only receive it once. I fed it to a weapon, and now there's no chance of getting it again. I recommend you refine 3-star weapons because some are specific to regions like White tassel found in Liyue.  So, pay attention to the rewards you get from NPCs because you may not know that it's a rare item to have in your inventory, and you may accidentally destroy it. My tip for you would be to interact with NPCs and learn about their stories, and who knows, you may get to know about a few lore pieces. There's also an achievement Friends the World over that you get after achieving a total of 10,000 dialogue option interactions when speaking with NPCs, and yes, I do have this achievement. Won't lie; I've ground the living breath out of this game.