OK let me adjust to realworld data so you can see what I'm doing. And then you can tell me I'm doing it in a stupidly complicated way 
Outbound City: ["Washington/Dulles", "Lome", "Addis Ababa"]
Arrival City: ["Lome", "Addis Ababa", "Johannesburg"]
Departure Time: ["9:55 pm", "12:55 pm", "11:30 pm"]
Arrival Time: ["11:55 am", "9:20 pm", "3:55 am"]
Airline: ["Ethiopian Airlines", "Ethiopian Airlines", "Ethiopian Airlines"]
Flight Number: ["517", "517", "859"]
Airline Number: ["Ethiopian Airlines 517", "Ethiopian Airlines 517", "Ethiopian Airlines 859"]
Booking Cabin: ["V", "V", "Y"]
Ethiopian 517 is actually a single flight that stops in Lome on the way to Addis Ababa. I'm cycling through a repeat for each item on the Airline Number and then doing a check to see if the data matches the previous item. If so, then I need to do some things, and if not, then I do other things.
{repeat: for segstosell in (seq(2, count(airlinenumber
)))}{if: airlinenumber[segstosell] <> airlinenumber[segstosell-1]}{=bookingcabin[segstosell]}{endif}
I can't just reduce that list, b/c it won't know which things in the other lists to ignore (though there's probably a way to reduce the duplicate position on each list type?) So as this sequents runs through it has 3 sequences to run though, but in this example it is outputting no data on the 2nd one b/c the airline number matches the first one. Then when it outputs the 3rd sequence of data I need it to be able to say this is the 2nd unique airline number.
Of note, the duplicate airline number could exist in any sequence of the list, so it isn't always 1&2.
Does that help or just make it murkier?