Currently hook_entity_uuid_presave() is invoked at the end of entity_make_entity_local() which is then called in entity_uuid_save(), just before entity_save(). In most cases this works out ok. However entity_make_entity_local() can also be called separately as it is in _uuid_services_entity_access(). So for example, when the uuid services update operation callback, _uuid_services_entity_update(), is called there is an access check which calls entity_make_entity_local() and invokes the presave hook. This is followed by the callback which calls entity_uuid_save() which again calls entity_make_entity_local() and invokes the hook a second time. So for something like a file entity the presave hook calls file_unmanaged_save_data() twice which is unnecessary at best.

I would suggest moving hook_entity_uuid_presave() to entity_uuid_save() and if it is necessary for entity_make_entity_local() to be alterable then a hook invocation specifically for that purpose should be added where hook_entity_uuid_presave() is currently called.