What makes an itinerary invalid is if it meets one of two conditions:
- It has overlapping flights in terms of times (i.e. two flights that are to be in the air at the same time.)
- Arrival and departure airports are not the same AND the departure airport is repeated within 12 hours.
This condition seems complicated, but its purpose is to identify when you're making the same point A to point B trip twice even if you are going via point C (e.g. PDX to LAS, then PDX to SEA and SEA to LAS) without invalidating co-terminal connections (it is perfectly valid to fly into LGA and out of JFK) or short business trips (e.g. SEA to PDX to LAX then LAX to PDX to SEA a few hours later.)