The ball hits a spare racket lying on the court. What happens?

Let: an accidental hindrance; replay the point. A spare racket on the court is a hindrance nobody meant: let, replay the point, and clear the court. A player loses the point if the ball touches anything they wear or carry other than the racket in their hand. A spare racket, a towel or a bottle lying on the floor is not being worn or carried, so a ball that hits one is treated as an unintentional hindrance: the point is replayed. Do it twice and a referee will start treating it as negligence. The practical lesson is the same either way: the court is for four people and four rackets. Bags, bottles and spares go outside the door, and a second ball stays in the server's pocket or goes back to the door as well.

The rule about touching a player covers the body, clothing and whatever they are holding. A racket lying on the floor is none of those; it is a hindrance, and a hindrance nobody meant is a let.

Nothing on the court except you, your partner and the ball in play. Bottles go by the door.

A ball that comes from another court onto yours is the same call: a let. The distinction the rules draw is intent, not who brought the object in.

The point is lost if the ball touches anything a player wears or carries other than the racket held in the hand. Interference from an object on the court that nobody intended is a hindrance, and the point is replayed.