Some further information to help you...
You can run an SQL query directly on the table to test if you have duplicate values...
SELECT `title`, COUNT(`title`) FROM `jos_finder_taxonomy` GROUP BY `title` HAVING COUNT( `title`) >1;
You can also replace the column name with any other. I tested 'path' and 'title'. Substitute all instances before running the query. The results will be shown in a table.
To fix the issue, you can also use the Joomla CLI interface to reindex the table...
In a SSH terminal navigate to the /cli/ directory of your Joomla installation, then run the following command
php joomla.php finder:index