Hi,

the only way to manage your case this is a comment line of Python code :

in file mrp/mrp.py line 264 :

  262.  _constraints = [
  263.   (_check_recursion, 'Error ! You cannot create recursive BoM.', ['parent_id']),
  264.  #(_check_product, 'BoM line product should not be same as BoM product.', ['product_id']),
         ]

but it is not professional, the best way is to create a new module to do that

To redefine a python constraint, you can override it by setting the same method function name:

In a module puts:

    class mrp_bom(orm.Model):

        _inherit = 'mrp.bom'

        def _check_product(self, cr, uid, ids, context=None):

            return True

        _constraints = [(_check_product, 'Override of _check_product constraint', ['product_id'])]