Dripping HVAC vent

On our rental property, each of 4 units has an Armstrong Magic-Pak "through-the-wall" combined HVAC system. They're 9 years old and haven't always been cared for properly; the one in question is a new replacement from January (heating died).
We had a 90 degree day here in Wisconsin, muggy and warm enough to buckle highway concrete in Madison. The ceiling vent closest to our tenant's HVAC has been dripping water. It's very clean, like condensate, and doesn't seem to be in the drywall at all (that's cool but not damp); it's coming right out of the ductwork and dripping through the vent device. This duct is well above the AC unit which is at chest level, and has exterior drip holes (which aren't dripping). The duct is on an east-facing exposed wall, but one that doesn't really get direct sun past noon or so. There's no nearby plumbing that I know of.
Before I rip out drywall, is this really just condensate? Why is it in the ducts? The HVAC itself appears to be working properly.
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Dan Hartung writes:

Bad design or bad performance can result in duct temperatures below the indoor dew point, yielding condensation and dripping from the exposed metal of the vents themselves where they contact the warmer/wetter room air.
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Is there a tenants Magic Pak unit above that duct that drips ?? IF so, maybe his condensate is overflowing down the inside of the wall and ending up on your duct , then dripping inside your unit.
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HVAC IsFun wrote:

Boy, that should have been obvious. I'll check that right away.
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