Purchased an item online. Received package. Following day got a duplicate item. I Called company that they made a mistake, they said they’ll retrieve it. They didn’t. What’s the law on returning item that’s not mine?
You can keep it.
Additionally there is an american law that anything sent to your house you may by rights keep. This is due to an old sketchy practice of mailing things to people and then sending a bill and expecting the person to pay. The Govt made a law that if someone sends something to you, it is yours, period. Just an FYI for the next time that at least by US law you probably don’t even need to call them, AFAIK - the Rov can contribute if the same goes by rabbinic law.
I found that out when I was a kid. Readers Digest sent me a book and then a bill. When I called to complain, they apologized and told me that by law I can keep the book and it was their mistake.
Local law sets the rule here.