Change in duplicate bookings policy?

I had long thought that BA didn't cancel duplicate bookings if they were ticketed, and had said this in many threads.

But I've recently had a contrary experience. I booked Avios seats on two LCY-EDI flights on the same day, one in the morning and one at lunchtime, with the intention of deciding later which one to take. Five months after booking, and three months before the flight, BA cancelled the later of the bookings.

There was no advance warning, nor (oddly) was there any communication after the cancellation; I only discovered the cancellation when the refunds appeared in my BAEC and Amex accounts.

I called BA to find out why. The agent told me that it was because there were duplicate bookings, and that this reason was noted in my booking. (I think that had to be true, because I had not already told her about the earlier booking.) I asked to be pointed to any rule that allows BA to do this. She couldn't, but said that this is normal BA practice.

Is it? Or perhaps: is it now? I think it would be contrary to what we'd understood before, but the agent was adamant that this was why it had happened. I've searched for recent discussion here about it, but can't find any.