Black Lion Chest Bug
You can contact the CS Team for assistance, and/or post in the ‘official’ thread asking for information about this ‘bug’.
Good luck.
https://forum-en.gw2archive.eu/forum/support/bugs/Black-Lion-Chest-duplicate-bug/first#post6417344
There hasn’t been a fix yet. Each time this happens to you, you need to contact support — they’ll send you replacement keys after asking you to delete the item. Alternatively, opening more chests will fix this too as soon as another duplicate spawns. Yes, I know, it’s weird and annoying.
To avoid this from happening, close the BLC window after each chest opening.
(edited by Ashantara.8731)