Having one flight number for sequential sectors such as BA15/16 makes perfect sense, In the case of Pan Am's round the world flight PA1/2 the flight could have about ten sectors, differing by day of the week.
An example of an out and back with with the same number is AA or MQ 3378 ORD - PIT - ORD for today (17 Nov) but there are numerous examples. Look it up on flightstats.com or aa.com. Presumably it is to identify to the airline that the same aircraft does the out & back, but that's just a guess.