diff --git a/lib/kolab_sync_data_calendar.php b/lib/kolab_sync_data_calendar.php
--- a/lib/kolab_sync_data_calendar.php
+++ b/lib/kolab_sync_data_calendar.php
@@ -345,8 +345,23 @@
      */
     public function toKolab(Syncroton_Model_IEntry $data, $folderid, $entry = null, $timezone = null)
     {
+        $foldername = isset($entry['_mailbox']) ? $entry['_mailbox'] : $this->getFolderName($folderid);
+        if (empty($entry)) {
+            // If we don't have an existing event (not a modification) we nevertheless check for conflicts.
+            // This is necessary so we don't overwrite the server-side copy in case the client did not have it available
+            // when generating an Add command.
+            try {
+                $folder = $this->getFolderObject($foldername);
+                $entry = $folder->get_object($data->uID);
+
+                if ($entry) {
+                    $this->logger->debug('Found and existing event for UID: ' . $data->uID);
+                }
+            } catch (Exception $e) {
+                // uID is not available on exceptions, so we guard for that and silently ignore.
+            }
+        }
         $event        = !empty($entry) ? $entry : array();
-        $foldername   = isset($event['_mailbox']) ? $event['_mailbox'] : $this->getFolderName($folderid);
         $config       = $this->getFolderConfig($foldername);
         $is_exception = $data instanceof Syncroton_Model_EventException;
         $dummy_tz     = str_repeat('A', 230) . '==';