No, you have been wronged but it is not worth shooting yourself in the foot over it. Don't give up on the therapy because of the payment issue afterall it's not your therapist that has made the mistake. Just be real calm and state that you have already paid it and yes make sure you get a receipt no matter what!
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Pegasus
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“Everybody is a genius. But if you judge a fish by it's ability to climb a tree, it will live it's whole life believing that it is stupid.” - Albert Einstein