How To Get User Data in Callback Function for pre_user_nicename?

Whenever I update a user profile in the dashboard, the user nice-name gets changed in the database: Every time, a “-2” is added.

I tried to reset the nicename (based loosely on this thread) by hooking into edit_user_profile_update and personal_options_update, but that seems to be to early, the value does not change:

add_action( 'personal_options_update', 'my_reset_nicename_function' );
add_action( 'edit_user_profile_update', 'my_reset_nicename_function' );

function my_reset_nicename_function( $user_id ) {
    $user = new WP_User( $user_id );
    $nice_name = $_POST['first_name'] . ' ' . $_POST['last_name'];
    $nice_name = sanitize_text_field( $display_name );
    wp_update_user( array(
        'ID'            => $user->ID,
        'user_nicename' => $display_name
    ) );
}

Then I read about the pre_user_nicename-filter (which feels like a much cleaner way), but any callback function filtering the nicename only gets the nicename as an argument, and one cannot get a user-object by its nicename only:

add_filter( 'pre_user_nicename','my_user_nicename_filter_function' ) );
function my_user_nicename_filter_function( $user_nicename ) {
    // How to get the user's firstname or lastname here?
}

So, how can I reset the user’s nice-name to his first- and lastname when saving his profile in the dashboard?

EDIT: When I deactivate my pre_user_login callback (to automatically define the user_logins), the above behaviour does not occur. Can anybody please give me a push?

If anyone should be faced with this problem, check if you have defined a callback function to set the user_login on pre_user_login. This filter is applied inside the wp_insert_user()-function and produced a same username again. Therefore the nicename was changed by adding a “-2”.

So, my problem was that I reset the user_logins again and again, instead of only once when registering.