This is overly dense and could do with editing but is pretty interesting imo. Here’s my attempt at summary:

• NATS gets the flightplan in ICAO format
• they need to extract just the UK part of that flight plan
• they convert the ICAO flight-plan to ADEXP format, which contains additional waypoints
• they find the UK section of the ADEXP
• they now need to convert back to ICAO
• they try to match the start and end of the UK ADEXP to ICAO
• duplicate waypoint names exist! The end of the ADEXP route gets wrongly matched with the start of the ICAO route (cos they have the same name)
• the software recognises this doesn’t make sense, but instead of rejecting this flight plan shuts the entire system down
• the backup system comes online, and reprocesses the same data, hits the same bug, and shuts down as well