Off screen Will Riker found his transporter clone Thomas Riker in a Klingon bar getting drunk after being rescued from the Cardassian camp during the Klingon Cardassian proxy war and was serving on a Klingon battlecruiser as first officer then captain during the Dominion war. At the bar they got into a scuffle and one was vaporised, the survivor walked away and we don't know which Riker it is to this day. So at the end of season 3 Riker will reveal he's actually Thomas Riker and has been since the Titan and Picard will then kick him to the curb and send him on his way.