What is minimum information required to identify if two people will be on the same flight? I asked this question on travel.stackexchange and got an amazing response but I was advised it better suits aviation.stackexchange and should ask here for a better response, so here it is. 

I have a set of users and their itinerary details. I want to figure out who all will be sharing a common flight. I do not want to rely too much on the "Departure time" as people could have entered an approximate time into the system. How can I figure this out with minimal information?

Initially, I thought two people with the same flight number and same departure date would be on the same flight. But the same flight can be used again on the same day. Would the combination of "Flight no + departure date + departure city" be enough to identify a unique flight itinerary?

What would be optimal information to identify this?

I also have to keep in mind that there could be people with multiple flights in their itinerary and if at least one flight is common with another passenger, that has to be shown too.
Yes, Departure City + Flight Number + Departure Date will identify a single flight.

Where you may have a problem is if flight 123 goes from AAA to BBB to CCC to DDD on a particular day, and passenger 1 gets on flight 123 at AAA and flies all three segments, getting off at DDD, while passenger 2 gets on flight 123 at BBB and off at CCC.  To identify that those two are together on flight 123 departing BBB, you'd need to either have passenger 1 enter all the segments of his flight, even though he doesn't change planes at (nor even really much care about) BBB and CCC, or you'd need information about all the flights from some source, so that you know that "flight 123 from AAA to DDD" goes to BBB and CCC in between.