Repetitive buying is a common trait in mild dementia. It's especially frequent when grocery shopping, but it can also happen when shopping at hardware stores, drugstores, or any kind of shop the person frequents.
Why it happens: The force of habit is strong, and memory is weaker, so your loved one may go to the store with the same shopping list in his or her head, outing after outing, forgetting what was bought last time or what's in the cupboards at home.
What you can do:
At home, weed through food items regularly to throw away what's outdated.
Give away multiples to reduce clutter.
Offer to shop together, making a list beforehand (and pointing out that no, she doesn't need any more ketchup).
In the store, let your loved one make his or her own selections, but discreetly remove from the basket (or at checkout) the things you know aren't needed. Or suggest that there's enough on hand at home, but you'll write the item on the list for "next time."
You might also offer to shop for the person, to minimize the number of store trips he or she makes.
Look into grocery-delivery services; many stores allow you to shop online and then pick up your order or have it delivered to your home.