In 1968, the brother of Robert A. Ferguson appeared beside him during a speech at a Spiritualist convention. 'Walter' was killed during World War II.
What is fascinating about this photograph is that it is a Polaroid and typically thought to be incapable of producing a double-exposure (though there have been double-exposed Polaroid photos). But, if it were a double-exposure, the difference in contrast would not be so evident.
Closer Examination: Famous Ferguson Ghost Photograph
Notice how bright white Robert Ferguson's ears are from the lighting and how shadowed Walter's face and ears are...not the same man.
The ears on the man behind (Walter) are close to his head while Robert Ferguson's ears prominently stick out from his head. We also noticed the width of each man's head seems to be different from its counterpart.