Got it, it’s something to do with the interaction with the “WPSSO Core Pro” plugin. Switch this off, and the behaviour is as expected. The WPSSO plugin has setting: “Apply WordPress Content Filters” yes|no (default no). With this switch set to ‘yes’ the double title and other fields appear. The WPSSO plugin author says:
The use of WordPress content filters allows WPSSO Core [Main Plugin] to fully render your content text for meta tag descriptions and detect additional images and/or embedded videos provided by shortcodes.Many themes and plugins have badly coded content filters, so this option is disabled by default. If you use shortcodes in your content text, this option should be enabled — if you experience webpage layout or performance issues after enabling this option, determine which theme or plugin is filtering the content incorrectly and report the problem to its author(s). Advanced users can also hook the “wpsso_can_filter_content” filter and return true / false to enable or disable this feature dynamically.
If I disable this switch, as per the default, then the problem goes away. However, as the WPSSO plugin author suggests, maybe this is something which you might consider investigating in your own (otherwise excellent) plugin?