I have seen a few times where there are two commercial airline flights with the same flight number and neither has an added letter added to the flight number and it creates an interesting pattern on flightaware as it gets position updates from both flights at the same time.
When more than one plane from our flight school is practicing an instrument approach, with similar call-signs, "Skyhawk ## Lima Uniform," and it just so happens they are both practicing the same instrument approach. It gets interesting when one pilot keeps taking the other pilot's radio calls.
I have also seen a similar situation once at SNA with a Southwest 668 (I don't remember the actual flight number) and an Alaska 668 on frequency. Tower notified them that there was more than one airplane with the same flight number on frequency.