In this case LH 4923 is a codeshare flight operated by NH 217 (All Nippon Airways); LH 715 really is a different flight.
You can tell flights are codeshare as they will depart at the same time, from the same gate – but with different flight numbers, as depicted on this picture:
(Source: Wikipedia)
Lufthansa and ANA have a “joint venture” which enables them to coordinate prices and schedules, and share revenues, on routes between Japan and Europe. Therefore the extra capacity was deliberately organized to capture the high demand for travel at this time.
It is quite common for flights to clump together and depart at around the same time, because if a departure time is popular with customers, then of course everyone wants to put their plane on at that time to charge higher fares.
They are different flights.
LH4923 is a Lufthansa codeshare for a flight actually operated by All Nippon Airways as flight NH217: http://www.flightstats.com/go/Mobile/flightStatusByFlight.do?flightNumber=4923&airline=LH
LH715 is an actual Lufthansa flight: http://www.flightstats.com/go/Mobile/flightStatusByFlight.do?airline=LH&flightNumber=715&departureDate=2017-03-14
It’s uncommon, but not unheard of, for this to happen: for example, all 3 daily flights from Tokyo to Sydney depart within a few minutes of each other, two of them from the same airport (Haneda).
Credit:stackoverflow.com‘