I think your mother-in-law wants to show her "importance" and "helpfulness". As long as she is happy, I think just let her do it. But on one hand you may change the method of dealing this issue. Instead of asking her not to buy so much, perhaps you may try asking her to buy what you want. Give her a list and ask her just limit to that or you won't take it because you don't need them. Also, there would not be any overlapping with those things you bought. In that case you didn't stop her from buying (she probably enjoy buying things for you). Also give her some "praising" whether she bought something good so that she knows even she doesn't buy that much she could be appreciated.
If I were in your shoes, I would just pick those I want, tell 99 friendly that for the rest, I still have many stock at home and want to use them up first (as most old ladies do not waste food and this can avoid from hurting her feeling as if being rejected thousand miles away- 被人拒於千里之外)