Please check the dashboard page from wp-admin if you have placed the shortcode twice.
The best way to modify something is via child theme. Child themes keep your theme and plugin safe. Because, child theme will never get an update and no changes would be lost, but plugins and themes do get updated regularly.
Please navigate to dokan->settings->page settings and check if you have set a page for dashboard.
And go to pages and check if you have multiple pages named dashboard. If you have multiple pages named dashboard please keep one and delete the rest.
If you have done the above and you are still getting the error then please check for theme or plugin conflicts. You can disable all the plugins except woocommerce and dokan and switch to a default WordPress theme like “Twenty Twelve” and check and let me know.