I experienced a conflict between Custom Post Widget and Event Organiser. After installing Event Organiser, event metadata was added to all my Custom Post Widgets, resulting in showing the same metadata multiple times.
Event Organiser calls the 'the_content' filter if you're not using a custom event template (for example: single-event.php).
When not using a custom template, Event Organiser will fallback and use your 'single.php' template and activate the 'the_content' filter to add the event meta data to the event.
When using more plugins that use this filter on the same page (for example custom post widgets), Event Organiser will also add the meta data to this plugin.