Foreign Keys to "Grandparent": Duplicate Data?

Let's talk about storing additional "duplicate" foreign keys, with an example from the open-source project Krayin CRM.

In the Krayin CRM project, there is a table product_inventories for which migration looks like this:

Schema::create('product_inventories', function (Blueprint $table) {
    $table->increments('id');
    $table->integer('in_stock')->default(0);
    $table->integer('allocated')->default(0);
 
    $table->integer('product_id')->unsigned();
    $table->foreign('product_id')->references('id')->on('products')->onDelete('cascade');
 
    $table->integer('warehouse_id')->unsigned()->nullable();
    $table->foreign('warehouse_id')->references('id')->on('warehouses')->onDelete('cascade');
 
    $table->integer('warehouse_location_id')->unsigned()->nullable(); 
    $table->foreign('warehouse_location_id')->references('id')->on('warehouse_locations')->onDelete('SET NULL');
 
    $table->timestamps();
});

The warehouse_location_id field is from a grandparent table. A simplified schema looks like this:

So, we have the location_id field in both the "child" and "grandchild" tables, duplicating data.

Technically, this violates...