site optimeindexes in set not list

essmsync/booker.py

@@ -260,8 +260,8 @@ def op_get_sites():
     if config.get('booker.buildings') is None:
         op_get_buildings()
     # Get all (non -1) site OptimeIndexes for filtered buildings
-    site_ois = [b['Site'] for b in config.get('booker.buildings')
-                if b['Site'] != -1]
+    site_ois = {b['Site'] for b in config.get('booker.buildings')
+                if b['Site'] != -1}
     if len(site_ois) == 0:
         LOG.error('No sites in Booker related to TermTime rooms')
         raise BookerAPIError()