You can't have two flight 70's in the air at the same time. If the original (late) flight overlapped the next day's same flight at all it would have to be changed. Remember, airlines may have the same flight number not just on the JNU-SIT segment. The flight into JNU might have been flight 70 with continuing service (and same flight number) into SIT. Some carriers routinely use four digit flight numbers. In this case the four digit flight number refers to the irregular operation. Airlines may also code a flight (i.e. 72 might indicate what transpired with the original flight 70).