Hi,

I had the same problem. I used a workaround. Odoo seems no accept new one2many lines with 'external id'. I solve this problem like that:

Imagine that you want to import this file:

On first import ignore the one2many_ids field, then for the next imports, odoo use the field one2many_ids field to find the right record by his name, and it doesn't create new repeated lines.

Unfortunately mrp.bom.line model doesn't have a field 'name', but you can inherit this model to add 'name' field. May be you could try using the field _rec_name = "product_id" .

Please if you  find this usefull, please post a good valoration