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OK so this is not a steam dryer model so there is no way that water from the dryer itself is leaking into the drum. That means your vent line going out your house is either shared with another vent line (which it's not supposed to be) or the damper on the outside of your house isn't closing properly to stop the air from blowing from the outside of your house back into the dryer. If these dryers share the same vent line with each other or some other appliance, the vent will need to be completely re-ran through your house. If they each have independent vent lines out your house, then the dampers on the outside of your house are not sealing up and will need to be replaced.
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Sorry but if all of that was checked and it was all working then you wouldn't have an issue---it's that simple. There are no other options so if you think it's all good, it's time to call someone else out who hasn't looked at the situation yet because you are whoever you called out the first time is overlooking something obvious that I have explained above.
Hello again. I'm just following up because I haven't heard back from you and I see this question is still open. Is there anything else I can help you with?