The database table in question has a unique username column. When the usernames are listed in phpMyAdmin or SqlYog, a duplicate for at least one username appears.
When the username is displayed on a web page and the HTML is inspected, it can be seen that the duplicate username is preceded by an HTML entity ‏ (right-to-left mark) which is not shown on the page because it has no visual representation or associated symbol.
Trying to search for the entity:
SELECT * FROM mytable WHERE username LIKE '%‏%';
returns no results, because the invisible symbol not the entity is stored.
When exporting to Excel from SqlYog, it appears as †in Excel.
Remove the visible characters that are normally in usernames from each username, then check which usernames are still not empty but appear empty.
SELECT username,
REGEXP_REPLACE(username, "[\\.,{}<>\'a-z0-9@_-]", '') AS second_username
FROM mytable
HAVING second_username <> '';