Description

Without this, redirects can cause Response::send() to be called multiple times, causing HeadersAlreadySentException to be thrown.

Example:

• A controller calls $this->response->sendFile()
• A listener on \yii\web\Response::EVENT_AFTER_PREPARE calls Response::redirect()
• That triggers another Response::send(), which sends headers
• The remaining calls to Response::send() (from $app->run) attempt to send headers, throwing HeadersAlreadySentException.