未捕获重复输入错误。 但它不重复

This is the error

Fatal error: Uncaught Duplicate entry '8-1' for key 'PRIMARY'<br /><br /> thrown in prestashop/classes/db/Db.php on line 613

The Database:

Column  Type    Comment
id_category int(10) unsigned     
id_group    int(10) unsigned    

Indexes
PRIMARY id_category, id_group
INDEX   id_category
INDEX   id_group

The thing is I am sure there isn't any duplicate 8,1 entry in the table. There is 7,1 7,2 7,3 , but no 8,1 in the table. The var_dump in the previous line is only shown once so its not being called twice.
I just don't get it . All I had done was manually truncate a few prestashop tables(using a custom tool), after which these errors have been coming. Now I ran the prestashop cleaner, but still getting this error.

Edit The sql insert statement:

>INSERT INTO `ps_category_group` (`id_category`, `id_group`) VALUES ('8', '1'), ('8', '2'), ('8', '3')