The first clue is usually on the porch. Two boxes of the same adult briefs, two jugs of the same bottled water, or a second grocery delivery in the same week means two people placed an order without knowing about each other. Nobody did anything wrong on purpose. The family just has no single place where orders get written down, so each sibling fills the gap from their own kitchen.
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What to check first
Look at who holds the login for the store account or delivery app, and who gets the confirmation emails. If three different phones are getting alerts, you have three people ordering from three different lists in their heads. Then ask three questions: Who placed the last three orders? Is there one shared list that shows what is already in the closet? Does the account reorder on its own, and if so, who turned that on and when was it last checked?
Spot it early
Count what is on hand once a week, on the same day, in the same spot. A shelf that keeps filling up with the same item is the early sign, long before anyone complains. Write the count in the shared note so the next person can see it without asking you.
Keep the fix small. Pick one person who places orders, one list that anyone can add to, and one rule: check the shelf before you buy. Put the rule in the group text so it is written down, not remembered.
Be fair to the one who ordered
The sibling who placed the second order may have been covering a gap nobody told them about. Maybe they thought the first delivery was missed, or they were shopping for the house while they were already in town. Ask before you assume. You are looking for a routing problem, not a villain, and a routing problem is fixed by a list and a rule.