In a transporter incident in 2361, Thomas Riker was formed. He is genetically identical to Will, which is a security threat to StarFleet, but never addressed in the series. He has the same personality and memories as Will up until 2361, that is.
Will was a lieutenant on the USS Potemkin when this happen. He was leading an excavation on Nervalla IV. The atmosphere had created a distortion field, which prevented the use of shuttle pods as well as transporter except once every eight years.
