I have noticed that on certain days there are two BA174 flights into LHR from JFK usually following each other.
Nope, there's only one scheduled BA174 per day so I suspect your tracking software is at fault.
Using the same flight numbers on the same day could potentially cause chaos with crew rosters, pax check-in and baggage handling, flight planning, engineering, fuelling etc.....
To avoid any such confusion in the rare event that you do end up with a major delay and a flight delayed by say 24 hours then as DR has said the usual solution is to stick a suffix to the flight number or change it completely.