This is a problem I have created for myself, of course. If I had diligently kept records of which of my roughly 9,000 favorite books I had previously bought for my sister, then I would at least have some idea what books not to buy for her again.
She has moved to a different continent twice recently, so there's a decent chance that if it's a book I bought for her over 10 years ago, she will no longer have it and has probably forgotten I bought it for her, AND YET.
But still. Scrolling through book lists looking for books that are Not Old Favourites is proving something of a struggle. I know she likes cosy mysteries, has some tolerance for historical fiction and fantasy, likes good writing, but is not a hard-SF person....
I used to be moderately good at finding books that didn't prominently feature covers of muscly torsos and where the writeup didn't immediately tell you about the protagonists' amber hair, but I am having no luck today. Perhaps tomorrow will be a better bookhunting day.