I would just ask her to return it , tell her you’ve already got it for him long ago so can’t be returned and you don’t want him to have a duplicate. I wouldn’t get into which version was better x
I'd also be annoyed at this. Maybe your partner let slip what he wanted? Is that what he means? . I'm guessing it's too late for you to say you already bought it for him?
I’d do something like @Natalie’s suggestion
Buy him his main present and give it to him first thing. Serves her right. X