How was the collision between 2 planes that were just 1.8 km apart averted at Delhi airport?

A mishap was averted at Delhi airport when a pilot intervened on time preventing two Vistara flights, cleared to take-off and land at same time, from colliding. Captain Sonu Gill, at the helm of the Ahmedabad-Delhi flight, spotted it when planes were 1.8 km apart and alerted ATC. ATC took control immediately and asked Delhi-Bagdogra flight to abort take-off.