diff --git src/wp-includes/user.php b/user.php
index 566702f..4a530b1 100755
--- src/wp-includes/user.php
+++ b/user.php
@@ -2439,6 +2439,23 @@ function register_new_user( $user_login, $user_email ) {
 	}
 
 	/**
+	 * Additional checks for cross-referenced email and username among existing accounts.
+	 * Prevent using existing email address as username and vice versa.
+	 * Do not allow an existing email address to be used as a username of the new account.
+	 * Do not allow an existing username to be used as the email address of the new account.
+	 */
+	
+	// Check if username is being used as the email address of another account.
+	if ( email_exists( $sanitized_user_login ) ) {
+		$errors->add( 'email_exists', __( '<strong>ERROR</strong>: This username is invalid because it is already the email address of another account.' ) );
+	}
+
+	// Check if email address is being used as the username of another account.
+	if ( '' !== $user_email && username_exists( $user_email ) ) {
+		$errors->add( 'username_exists', __( '<strong>ERROR</strong>: This email address cannot be used because it is already the username of another account.' ) );
+	}
+
+	/**
 	 * Fires when submitting registration form data, before the user is created.
 	 *
 	 * @since 2.1.0