I bought a piece of urgently needed computer equipment and paid for next day delivery. It didn't arrive, and I raised hell with them, so they sent another, but it was still late.
Two items arrived, needless to say.
They had advised me that the first item had got lost in the delivery system, refunded my payment, and took a payment for the replacement.
Because they had caused me so much hassle, and made me miss a deadline, I thought why should I make more phone calls and waste any more time.
So I stuck the extra item on one side, and waited for them to sort it out.
That was 3 months ago. The item cost about £100, and the supplier is a massive organisation.
Stick it on one side and in about a year when you had forgotten it was there you will find it and then either sell it or give it away. You can always say you were expecting them to collect it if they ask - which they are highly unlikely to do.
Leave it a couple of months then flog it on ebay. Considering the big corporations get kids to operate their assembly lines in third world countries, I'd say its ethically acceptable to deprive them of their assets.
3 months ago I'd have sent it back, that was when any moral dilemma may have arisen. It's too late now so do what you want with it and forget about it.
I don't think it matters that the supplier is a massive organisation or the value of the item - I think it matters how you feel about it deep down. If you have no problem with keeping it then do so - if it concerns you then return it.
I had a similar problem with some GHD hair straighteners once although it was a set I had returned to be repaired that didn't show up so they sent me a new set. The next day the first set arrived. As my receptionist at work signed for one pair then I kept them both and flogged on set to a woman at work. Do I feel guilty? Nope.