Wouldn't these faults be evident in the "viewfinder" mode (before
taking the exposure)? When I aim the camera at light and dark
subjects the camera compensates by "irising" up and down to
give what looks to be a properly-exposed "preview" display.
Only when the image is captured is it overexposed.

Images downloaded and viewed on the computer are overexposed,
identical to when viewed on the camera's display.

This isn't what I remember you saying. Regardless...

If the picture is consistently misexposed, then the exposure-compensation
control (assuming the camera has one) should fix the problem. If it doesn't,
then the camera needs repair or replacement.

As I said, this discrepancy is not uncommon.