Since return air ducts are usually before before the filter (which is the case when your air filter is at the air handler or furnace) you will almost always have dust in the return duct. This dust will not be able to get back into your airflow as the filter should catch it, so don't be scammed about dust in the return air ducts. Some systems have the air filter right at the return grill and in this case should prevent most of the dust from entering the duct.
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