On second thought, the digital processing should
eliminate any such effects on the RF side.

I, too, am confused. A sufficiently strong ghost should garble the data to
the point where the image is corrupted -- not duplicated.

My guess is that the "ghosting" has some other source. It might even be in
the signal itself. If the low-pass video filter is not properly
phase-equalized, there can be severe ringing on the trailing edge of
objects, which would appear as "ghosts" of vertical lines.