Tipp: This Contract is a good Target for the Even Odds Achievement also this Achievement is missable!
You get this Contract Named An Elusive Thief (Level 13) in Novigrad from the Notice Board in Hierarch Square. Go to the Marketplace during Day and Talk to Sylvester Amello. Then use the Witcher Senses to the Thiefs House and follow his trail again.
Save before the fight and kill the doppler and collect your reward (There you get the Achievement). Now you may load and let him live, because he gets your a good Diagram for a Weapon: Gnomish Gwyhyr Steel Sword (if ever hit Level 42 :D)