Does anyone know if the new UA frowns upon duplicate segments, in two different classes of service?

Scenario: I have a Y to C upgrade that's unlikely to clear, but I could book a free F seat on the same flight... I'm aware that checking in might be tricky but it may just work... After all I'd be paying for both tickets.

The old UA was running a watchdog to detect and cancel duplicate segments, but the algorithm was rather limited and there were multiple scenarios it would fail to catch. Any experience with the new UA?