inline_entity_form_process_entity_form() creates duplicate entities when validation error requires "Save" be clicked twice.

To recreate this issue:

1. Add two fields to a node, 1 Entity Ref for products, one a basic text field that is required
2. Add a new node of this type and in the add form, leave the required text field blank
3. Click the "Add new product" button and fill out the product form
4. Now click the parent "Save" button, not "Create product". This will leave the product form open
5. You should get a required text field validation error
6. Click "Save" again while leaving the required field blank
7. You should get another required text field validation error
8. Now fill in the required field and click "Save" again
9. You either just created 3 identical products, or you will get a PDO error for duplicate product SKUs

This is caused by the inline_entity_form_process_entity_form() being called before the required text field has failed validation. Each time inline_entity_form_process_entity_form() is called on Save, you are added a new entity to the $form_state['inline_entity_form'][$ief_id]['entities'] array.

To fix this problem, I've added an "open" flag set to true on inline_entity_form_open_form() and false on inline_entity_form_close_form() for the multi-entity widget. Then inline_entity_form_process_entity_form() check to see if the main parent "Save" button was the triggering element when the form was open. It now forces you to click "Create product" BEFORE you can click "Save" on the multi widget. For the single IEF widget, it checks to make sure there are no entities on the $form_state['inline_entity_form'][$ief_id]['entities'] array before it adds one.

Patch to follow.