In Revelation 14:16 the son of man reaps the harvest of the earth because it was ripe. Are these the people that denied Jesus that are reaped? Then in 14:18 an angel is told to reap the “clusters of the vine” of the earth. Are these two harvests the same thing, and why does the angel need to repeat the harvest if it is the same thing?
These are two prophecies of the coming bowl judgments which begin in Rev. 15. The dual purpose of these judgments is to completely destroy the nations of Earth and to discipline Israel in preparation for the Kingdom Age (Jeremiah 30:11). This may be why the prophecy is given twice.
Another possibility comes from Genesis 41:32. Pharaoh had two different dreams about seven plentiful years followed by seven years of famine. Joseph said the two similar dreams meant the matter had been firmly decided by God and would take place soon.
Since the Bowl Judgments begin in the following chapter of Revelation, this interpretation also fits.