Without additional details re: your trips, I'd say that you have already paid the departure tax for both trips (twice, if you will). 700 THB for BKK-TPE-BKK and 700 THB for your NW itinerary.

The airlines choose to collect the Departure Tax as they are responsible for paying AoT now, and they know if you have already paid it based on the fare details. There could still be many tickets out there, issued before the increase and automatic inclusion, so they continue to monitor the situation. I had a departure in late September and had to pay the 700 THB at check-in as my ticket was issued before the tax started to be included in the fare. The exact USD amount, or whatever currency was used, will vary as the exchange rate. You can request a fare breakout, I believe the Departure Tax code is "ST", at least it is on my receipts.