Preconditions

Magento version 2.1.7
Updated(09/19/2019): Still actual on 2.3-develop

Steps to reproduce

1. Create a Grouped Product
2. Add child / associated Simple Products to the Grouped Product
3. Save the Grouped Product
4. Duplicate the Grouped Product

Expected Result

The duplicate of the Grouped Product has the same associated Simple Products as the earlier created Grouped Product.

Actual Result

The duplicate of the Grouped Product has no associated Simple Product.

Investigation

On duplication the Magento\GroupedProduct\Model\Product\Type\Grouped::beforeSave function is called. In this function the cache for the associated products is being cleaned:

After removing this line of code, the duplication of Grouped Products works as expected.