What is minimum information required to identify if two people will be on the same flight? [closed]

To fully answer this question you need the administrative operating carrier and flight number, scheduled departure date, departure airport, stops and arrival airport. If all of these are the same the flight will be shared.

You specifically need the administrative operating carrier as there may be multiple marketing carriers for a single flight, so if you look at the marketing carrier and flight number you may get false negatives.

The arrival and destination airport are needed to determine whether the two itineraries overlap on the flight number, as a flight number may consist of multiple stops, where people are sold some subset of the stops. There are flights in Alaska with 6-7 stops, with people getting off and on at each stop. This also means that the question is not yes/no, but yes/no/partially.

You suggest that people are going to be entering information into the system. Most people will only know their marketing carrier and flight number and not the administrative operating carrier. As such, you may have to source a timetables dataset to map marketing flight numbers to administrative operating flight numbers.

I once flew from Auckland International to Great Barrier Island, and upon checking in different passengers were given different coloured boarding cards (as far as I know each group was given only one colour). These corresponded to different aircraft that we would be travelling on, even though we had nominally the same flight number, departure time, departure airport, and destination airport.

So, I do not think there is a general answer to your question.

You've really got two separate questions here - one of which is potentially a fit for here (although Aviation.SE might be a better fit). The other belongs on one of the more computer science SE sites...

Firstly, around the minimum set needed to know if two people are on the same flight. With some very corner case exceptions (eg, very small aircraft flights that run "on-demand" rather than being on a true schedule), the combination of flight number, departure city, and scheduled departure date will give you a unique combination. (I specifically call out scheduled departure date, as the actual departure date could change due to a delay, at which point these details will not be unique).

The need for the flight number and departure date is hopefully obvious. The need for the departure city is required because, as you called out, flight numbers can be reused even on the same day.

With some very (very!) small airlines there may still be conflicts, but these will be at least extremely rare, and short of adding in schedule departure time (which you've said you don't want to do) will generally not be something you can resolve.

The second part of your question around how to actually detect two people are on a common flight, given the information above, is a question for another site in the StackExchange network, not this one.

Airlines will keep flight numbers unique per day and departure airport (not city - some cities have multiple airports). The ATC system can't handle two planes in the air at the same time with the same flight number.

So you need to look at the combination of
date-flight #-departure airport.

This assumes that flight# includes the airline code.