A7RV producing multiple identical images of moving subject - how?

[RichardGB]

My A7RV should be recording 1x RAW and 1x JPEG of an image simultaneously in each media slot. However, it recently started recording 3 copies of the same image, each with a different filename, and each filename having a RAW and JPEG file. Not only is this using up space on the card but its also takes longer to clear the buffer.

The example images below are of successive OOC JPEG files. The train was probably moving at about 30km/h (>8m/s), so fast enough to show any movement if there had been any time difference between exposures.

As far as I can see, the images are identical in time - where the subject is moving there is no difference in position. They may have slightly different brightness, but the metadata for exposure is exactly the same. As far as I know, I haven't accidentally held the shutter release longer than a single shot, and in any case I would have heard multiple exposures.

I have now double-checked that the camera is set to Single Shooting, i.e. no Bracketing and no Cont.[inuous] Shooting, which would have given multiple exposures, and there is now only 1x RAW and 1x JPEG recorded - as there should be.

It is a mystery how the multiple identical exposures arose. Any suggestions?