Originally Posted by
lucky9876coins
This is actually quite normal. Since the flight was late another flight may have been in the air with the same flight number, which is likely the cause. Otherwise, there may have been an aircraft with a similar callsign, but it is usually the first instance.
It appears you may be right, as flight 597 has a MHT-->ORD leg that landed 20 min after we took off. This forum has led me to believe that one of the uses of "United xxx heavy" was when 2 flights with the same number are in the air simultaneously (which apparently is incorrect), so I hadn't considered this. Thanks.