How can I use a duplicate sku in Woocommerce?

To do this you’ll have to disable the unique SKU check.

add_filter( 'wc_product_has_unique_sku', '__return_false' ); 

This code needs to go into the functions.php file of your active child theme (or theme) folder. You can also create a plugin that has this code in it.

If you want to completely disable the SKU feature, you can use this code instead. It will remove the SKU field in both the backend and frontend of your store.

add_filter( 'wc_product_sku_enabled', '__return_false' );