[User Warning] Cache-Control header has already been set. Please use HTTPCacheControl API to set caching options instead.

Source

371 		if(headers_sent() && !$body) {
372 			return;
373 		}
374 
375 		// Warn if already assigned cache-control headers
376 		if ($body && $body->getHeader('Cache-Control')) {
377 			trigger_error(
378 				'Cache-Control header has already been set. '
379 				. 'Please use HTTPCacheControl API to set caching options instead.',
380 				E_USER_WARNING
381 			);
382 			return;
383 		}
384 
385 		$config = Config::inst()->forClass(__CLASS__);
386 

Trace

• trigger_error(Cache-Control header has already been set. Please use HTTPCacheControl API to set caching options instead.,512)
  HTTP.php:380
• HTTP::add_cache_headers(SS_HTTPResponse)
  Director.php:389
• Director::handleRequest(SS_HTTPRequest,Session,DataModel)
  Director.php:147
• Director::direct(/swimming/calendar/ics/84/20210207T083000-20210207T190000,DataModel)
  main.php:206