Just to clarify, according to the links posted further up this thread, duplicate items CAN’T be treated as unconditional gifts
So the law is NOT on your side.
As you say you want to have clarity, your first statement is correct, but your second is not.
If someone sends you an extra item by mistake, you have NO duty to pro-actively tell them, and you especially do not have to send it back (they have to pick it up).
You probably have an obligation to kekep it safe (a bailee’s duty of care) but only for a reasonable time.
What you may not do is profit from the situation.