Hi,

You may try like this:

err_products = []
for line in self.line_ids :
    if line.product_id.id not in err_products:
        if line.quantity > 0
            self.action_picking_create()
        else :
	    raise exceptions.Warning(_('Quantity must be > 0'))
	    err_products.append(line.product_id.id)
    else:
        raise exceptions.Warning(_('Product Exist'))

Many Thanks to Akhil.

product_id = False product_ids =[] 
for line in self.line_ids : 
product_id=line.product_id.id 
if not ( product_id in product_ids) : 
product_ids.append(product_id) 
    if line.quantity>0 : 
       self.action_picking_create() 
    else : 
        raise exceptions.Warning( _('Quantity must be > 0')) 
else : raise exceptions.Warning( _('Product Exit' ))

Dear,

To check exist the same Product in the line level.

from odoo import models, fields, api
from odoo.exceptions import ValidationError


class PurchaseOrder(models.Model):
    _inherit = "purchase.order"

    @api.multi
    @api.constrains('order_line')
    def _check_exist_product_in_line(self):
      for purchase in self:
          exist_product_list = []
          for line in purchase.order_line:
             if line.product_id.id in exist_product_list:
                raise ValidationError(_('Product should be one per line.'))
             exist_product_list.append(line.product_id.id)

Hope this code help you

Best Thanks,

Ankit H Gandhi.