Check if web root has already been appended

lib/private/urlgenerator.php

@@ -149,6 +149,11 @@ class URLGenerator implements IURLGenerator {
 	public function getAbsoluteURL($url) {
 		$separator = $url[0] === '/' ? '' : '/';
 
-		return \OC_Request::serverProtocol() . '://' . \OC_Request::serverHost(). \OC::$WEBROOT . $separator . $url;
+		// The ownCloud web root can already be prepended.
+		$webRoot = substr($url, 0, strlen(\OC::$WEBROOT)) === \OC::$WEBROOT
+			? ''
+			: \OC::$WEBROOT;
+
+		return \OC_Request::serverProtocol() . '://' . \OC_Request::serverHost(). $webRoot . $separator . $url;
 	}
 }