diff --git a/inc/caldav-PROPFIND.php b/inc/caldav-PROPFIND.php
index 21a28e77..887ec5de 100644
--- a/inc/caldav-PROPFIND.php
+++ b/inc/caldav-PROPFIND.php
@@ -105,7 +105,9 @@ function get_collection_contents( $depth, $collection, $parent_path = null ) {
   global $c, $session, $request, $reply, $property_list;
 
   // for http header comparison
-  function compare_val_with_re($val, $re){ return preg_match($re, $val)===1 ? 0 : 1; }
+  if (!  function_exists ('compare_val_with_re') ) {
+      function compare_val_with_re($val, $re){ return preg_match($re, $val)===1 ? 0 : 1; }
+  }
 
   $bound_from = $collection->bound_from();
   $bound_to   = $collection->dav_name();