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Duty is not the same as sales tax. Sales tax is a local, city burden on a business and varies from type of business, type of item, value of item and the state you are in.
A duty (more specifically customs duty) is a federal restriction and this applies universally across all borders of that country.
So - it is perfectly fine to buy an item with a sales tax that has no duty, and of course an item without a sales tax that is subject to duty.