thats quite unusual, you could try getter some visitors to go in and out of your house over and over again through out the day and then everytime he barks and whines spray him with a lemon spray (not too much lemon mind you because it's an acid). or you could try getting his attention away from the visitors while they leave by putting him in another room or in the garden, or possibly even throwing a ball for him to chase while you say farewell to them?

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