diff -r 312349b3712e -r 8d7d4a67820f misc/migration/3.5.3_Any.py
--- a/misc/migration/3.5.3_Any.py	Wed Oct 07 20:58:59 2009 +0200
+++ b/misc/migration/3.5.3_Any.py	Wed Oct 07 21:59:22 2009 +0200
@@ -1,4 +1,8 @@
 sync_schema_props_perms('state_of')
 sync_schema_props_perms('transition_of')
 
-add_attribute('BaseTransition', 'type')
+# type attribute might already be there if migrating from
+# version < 3.5 to version >= 3.5.3, BaseTransition being added
+# in bootstrap_migration
+if not schema.eschema('BaseTransition').has_subject_relation('type'):
+    add_attribute('BaseTransition', 'type')