The email addresses in Gerrit must be unique across all account, that is enforced on account creation and Gerrit would reject any such duplicate.

Today when I sending a review on https://gerrit.wikimedia.org/r/c/integration/config/+/1193304 , I wanted to notify @QChris. I went to write @qchris, pressed ⏎ to pick the first proposal then sent the reply. A modal window opened up with the following error:

Error undefined: Account 'christian@quelltextlich.at' is ambiguous (at most 3 shown):
79: QChris <christian@quelltextlich.at>
994: Wp-zero-data-bot <christian@quelltextlich.at>
christian@quelltextlich.at does not identify a registered user or group

Account id 994 is an old account created at a time when Gerrit most probably did not enforce uniqueness of accounts.

This task is to investigate whether we still have such duplicates and find a solution for them (which could be simply about disabling them).