Another Drain Issue
One of my techs (Jim Walch) mentioned to me that another common “double trap” style issue that comes up often is techs and installers running a drain too far into a condensate pump.
When you run the system drain too deep into the pump reservoir the water level can rise high enough to cover the drain end. This can create the same type of “air lock” you get with a double trap.
When draining into a condensate pump only extend the drain tube 1″ or so into the reservoir of the pump.