Someone dares besmirch the Witcher name? Deal with them!
This quest only opens up five days after you've completed the Witcher Contract The Merry Widow.
You need to be in the village of Lindenvale when the Ealdorman will start hurling abuse at you. Talk to him and he'll tell you a Witcher has been hanging around, shagging the locals and drinking all their booze. You better agree to find this Witcher and have words.
Go east of the village up the path to the top of the hill where you'll see a small chapel. Clear the grounds of ghouls and wraiths, then use the Witcher senses.
Follow the footsteps on the ground to find a chap near the coffin. He'll admit to being a fake Witcher. At this point villagers will arrive and you get to decide his fate.
You can tell them he's the real deal and it took both of you to dispatch the local horrors, or you can hand him over for punishment.
The quest is complete!