Am I entitled to keep the second item that reached me after I received my refund, or must I return its value?

Since you have received the refund, you are not entitled to benefit from the goods, neither the original nor the ones you received by mistake, unless the seller gives you permission to do so. Otherwise, it is obligatory to return the goods (the two watches and the two keyboards), and the seller must bear the cost of return, or you can purchase them from him. Receiving goods by mistake does not permit benefiting from them after the refund unless the owner gives his full consent, as Allah Almighty says: "O you who have believed, do not consume one another's properties unjustly, but only [in lawful] business by mutual consent." And as the Prophet, peace and blessings be upon him, said: "The property of a Muslim is not lawful except with his willing consent." Therefore, the seller must be informed of what happened.