If the file /etc/maxscale.cnf.d/maxscale.cnf exists and it contains a [maxscale] section, the file is loaded as if it were a persisted file when it isn't. If the file contains no errors, it is successfully loaded after which the normal file processing proceeds to detect a duplicate section which then causes a misleading error message to be logged. The correct thing to do would be to only use the global config handler for persisted files called maxscale.cnf and only run the global config handler for them, not both.