Multiple categories per product error

About my OpenCart Version 2.1.0.1

When trying to select multiple categories for a product I get a mySQL error reporting the product_id is already used as primary key thus can only link to one category. Tried importing with Excel for multiple categories and that didn't work either.

Trying to add to sub categories as well as parent categories to have products show up multiple times. Also tried to add to two different categories that were not linked to no avail.

Seems product_id is used as primary key.

This is the error I receive:
Notice: Error: Duplicate entry '2000' for key 'PRIMARY'
Error No: 1062
INSERT INTO oc_product_to_category SET product_id = '2000', category_id = '138' in /home/luplehe7/public_html/system/library/db/mysqli.php on line 41Warning: Cannot modify header information - headers already sent by (output started at /home/luplehe7/public_html/admin/index.php:81) in /home/luplehe7/public_html/system/storage/modification/system/library/response.php on line 12

Please help.