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 “airlock” 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 to prevent the water from rising and covering the end. Also, make sure to wire up the overflow switch on the pump in series with your system condensate switch so that whether the pump itself fails or the system drain pan has a blockage it will shut the system off.
Simple stuff, but it can prevent thousands of dollars in damage.