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If the landlord and tenant agree to enter into a rental contract, either verbal or written, and the landlord gives the tenant the keys, then that is legally giving them possession and the only way the landlord can regain possession is if the tenant voluntarily gives it back or the landlord evicts them for some breach of the lease.
So no, he can't give a tenant the keys and then rent the place to someone else or the tenant can sue him for breach of contract for any damages that they have suffered due to his breach.