It isn't necessarily the only one. When you have it serviced next, ask
the tech to check the air handler or furnace for a factory installed
filter, and to check for an electronic filter in the duct system.
Normally we wouldn't do this unless we find there's an airflow
restriction. I've found second and even third filters on systems that
the customer was completely unaware of, so yes, it is possible that you
have another, but it isn't necessarily likely. Just the same, if you're
concerned about it, have the tech take a peek for you. HTH.
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