During the conversation, he points towards a wall; the player should go to it and work his magic. When reaching the bridge, Geralt will notice that the trail dies. You should activate the Witcher sense near the stream in order to discover the footprints and follow them to a house located nearby.
After reading the note inside the house, you will discover that it is a Doppler, capable of changing appearances. The guard will reveal his true Doppler identity when provoked and it will turn into Geralt, triggering a fight.
Use Relict Oil, Northern Wind Bombs to deal more damage along with IGNI, AARD/AAXI signs. Defeating him will reveal two choices. Picking any one of them will ensure ‘The Doppler Effect’ Achievement Trophy.
Stay tuned for more updates on The Witcher 3: Wild Hunt!