Something old, new, blue...

I would maybe tell her that you're trying to plan out your accessories, and would really appreciate knowing what the something blue is ahead of time so you don't spend money on something else that the something blue would replace. Acknowledge that it is kind of her to want to surprise you, but you would prefer to know ahead of time. Maybe she could give you the something blue item on a special occasion day (birthday, anniversary, etc) sometime before your wedding?

I agree with the previous poster. My MIL didn't want to do a formal tea ceremony, so when she and my FIL surprised me while I was mid- dressed, I had one button done on my long shirt and I was in a thong. My husband told her to "just drop by" (eye roll). Because I was pressed for time, I thanked her and put her bagged gifts in the hotel safe. I never opened it and I didn't know it was 18k Chinese wedding jewelry until the day after. I wear it to all our get togethers now.

I'd be wary about that too. I like the idea of telling her you don't want to buy a duplicate.

It may be something you can tuck away into a pocket or your bodice or something (e.g., I was given a blue embroidered handkerchief) or even be a garter, which won't show anyway. Ask your future spouse to ask FMIL for more details, maybe, but I don't think I would worry about this. If it's terrible, you can just "forget" it in the final flurry of getting ready.