If you add a new page and change the template, you can click on one of the save buttons (_addanother, _continue, _save) before the page if fully loaded, which results in 2 pages being created.

This behavior could be prevented, if you disable all the save buttons right after the line:
form_element.find('[type=submit][name=_save]').click();
in the on_template_key_changed function in item_editor.js or you could render a loading screen to prevent the user from working on the page.