The Greek Coffin Mystery — Ellery Queen

From the very beginning, the Khalkis case struck a somber note. It began, as was peculiarly harmonious in the light of what was to come, with the death of an old man. Georg Khalkis, internationally famous art dealer and collector, died of heart failure. After his funeral, his attorney found that the will was missing and immediately called in the district attorney.

When Inspector Queen and his son, Ellery, are brought in to solve the mystery of the missing will, Ellery mentions the one place they have not searched for the will...the coffin! Upon exhumation of the Khalkis coffin they find that it contained not one body -- but two!