It's not a major bug. However, after finishing up the quest and receiving the reward from Kaenra. If I attempt to talk to her again she will continue to ask about "What happened with Purnisc? Did you give him the ring?"

 

The steps I took through the quest was

 

1. Talk to Kaenra

2. Talk to fake Purnisc

3. Go upstairs and talk to real Purnisc

4. At this point I diverged somewhat and went back to talk to Kaenra to see if I could somehow get some assistance before confronting the wizard. At this point she would say the same lines of "What happened with Purnisc?" So I just assumed the developers didn't think someone would try to talk to her before the confrontation.

5. Killed the wizard

6. Talked to Purnisc upstairs

7. Talked to Kaenra and finished the quest.

 

I would upload the log but it just gives me an "This upload failed" error.

I do also recognize this bug. It is possible to get the reward amulet as often as I speak to Kaenra.

I did the same as LilSassy except I didn't return to Keanra but killed the wizard directly after speaking to real Purnisc.

 

I realized another thing: I finished the quest before the patch 1.03 was released. If I remeber right, Kaenra left the tavern after I spoke to her. Then I somewhen installed the patch and continued playing. I was surprised seeing her a little while ago and being able to complete the quest because I was sure I already finished the quest.

Yep, the same for me. I can talk to her again and again after "finishing" the quest, and every time she gives a new amulet.