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You sound like a thoughtful person that believes in people doing the right thing and that is why it bothers you so much. If you can let it all go and just give this woman service with a smile that would serve you best. Even if you dig through 6 bags of trash and find that she is lying, it doesn't serve you well and I am thinking of you and your job. Next time she comes in before her order is given to her, go over it out loud and confirm that the order is correct so she can't do her usual thing.