In its current implementation, chrome://settings/passwords assumes the pair of (website, username) to be unique for each entry. While this is mostly true, this assumption breaks in corner cases, see e.g. https://crbug.com/890133.
https://crbug.com/865458#c2 contains some ideas how this could be achieved instead.
Comment 1 by jdoerrie@chromium.org
, Oct 4