This is most likely not sensor ghosting. The strongest explanation is a long exposure combined with flash.

With flash, the burst is very brief and freezes the main image. But if the shutter stays open for 1/5–0.3 seconds, any ambient light can also record before or after the flash. If the fly moves slightly during that time, you get a faint secondary image while the flash-lit image remains sharp.

That fits your settings: in aperture priority with flash, the camera may choose a relatively slow shutter to include ambient light. On Canon bodies, the flash sync setting in Auto can allow that behavior.

A reflection/optical ghost is less likely, though one answer noted it can resemble one. In practice, the long shutter time is the key issue here.

To reduce it, use a faster shutter speed up to your camera’s normal flash sync limit, reduce ambient light, or use manual exposure so the flash does most of the work.