No not the same as picking up cash on a public street. That is you intervening and taking something that was in no way directed to you, that could otherwise have got back to the owner.
This is an item being put on your property, intentionally. You have in no way intervened or taken action to make that happen. It has happened because the company and the courier have failed to do their job and already caused significant inconvenience to the customer- the stress of not getting the item when promised, the hassle of contacting the company to chase it down, and the hassle of finding an alternative product elsewhere. And now the hassle of receiving something you may not need anymore.
People don't seem to understand that the refund is to compensate for that and acknowledge that they failed to deliver the product in a timely manner. The company are effectively writing this cost off, and taking the financial hit in order to take responsibility for their mistake and maintain customer loyalty for the next time and hopefully avoid a stinking online review, they don't care about the product. Any decent company have a budget line for dealing with these situations, and if the cost of all these screw ups grows beyond what is tolerable for them, then that pushes them to make improvements - eg to adjust their time promises to be more accurate or employing more reliable couriers, or improving their processes so they are shipping things more reliably. It is not for the customer to 1. Take the hit if their inefficiency, then 2. proactively help them get their property back!
Yes, if the company explicitly said on offering a refund, that if the customer accepts, and the item turns up they would need it back , then fine , customer should accept it turn down these terms and stick to any agreement. And no, if asked if it turned up, customer should tell the truth. But companies don't do this because they don't care. It's more hassle than it's worth: (instructing customer in returns/ sending courier, risking pissing of already badly served customer, receiving and repackaging goods etc). When a delivery goes wrong, it's a sunk cost and the priority is to maintain customer loyalty for the long term.
It is not the customers job to bear the hassle of their screw up.