Why does the FOSUserBundle User entity have two properties for a username (and other fields)?

/**
 * @var string
 */
protected $username;

/**
 * @var string
 */
protected $usernameCanonical;

I've read the docs, browsed the source code, and tried to search for information, and I can't find out why it exists. It seems to always store the same values. I'm sure there is a good reason, can someone fill me in?

php symfony
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