Intruders in Black Quest-Reward brocken?

Maybe I accidentally collected a second item "Baleful Gem" after I finish the quest -
while I walked a second time in this area.

While the gem is in my inventary Helena won't interact with it,
but if i drop the gem in any other area, she yells around that I have to finish this quest while it is already finished.

Reference number is 2611068711

Thank you in advance for any hints.