You can add a clone of existing primary line items to the cart. The new cloned line item has the same selected configuration as the source line item. You can then modify the cloned line item as per your requirement. This eliminates the need for you to start configuring a line item from scratch.
Every line item on the Cart details page has a checkbox beside it. Selecting the checkbox opens a drawer with a list of selected items. From Select action, select Clone and click Confirm to add the selected items along with their configuration to the cart.
You can also Select All to select and add all the line items or remove some items and then add the rest of the items to the cart. You can later make modifications to the cloned line items.