diff --git a/lib/ext/Syncroton/Command/Sync.php b/lib/ext/Syncroton/Command/Sync.php
--- a/lib/ext/Syncroton/Command/Sync.php
+++ b/lib/ext/Syncroton/Command/Sync.php
@@ -465,6 +465,21 @@
             $this->_collections[$collectionData->collectionId] = $collectionData;
             $this->_modifications[$collectionData->collectionId] = $clientModifications;
         }
+
+        // Handle Calendar folders first.
+        // Outlook can't match MeetingRequest with an event that wasn't synced yet, which
+        // leads to duplicated events in Outlook's calendar.
+        uasort(
+            $this->_collections,
+            function ($a, $b) {
+                $ac = $a->folder && $a->folder->class == Syncroton_Data_Factory::CLASS_CALENDAR;
+                $bc = $b->folder && $b->folder->class == Syncroton_Data_Factory::CLASS_CALENDAR;
+                if ($ac) {
+                    return $bc ? 0 : -1;
+                }
+                return $bc ? 1 : 0;
+            }
+        );
     }
 
     /**