product->product.py

find class product.product add following code line no 588

_sql_constraints = [
        ('unique_product_barcode', 'unique(ean13)', 'Product Bar-Code  Must Be Unique .'),
     ]

upgrade this module Restart your server Settings->Installed Modules->find Products & Pricelists upgrade

OR

if you want to create separate module for this one follow this step

create one folder name should like

unique_product_barcode

inside this folder add __init__.py , __openerp__.py and yourmodule.py file

1) __init__.py

import unique_product

2) __openerp__.py

{
    'name': 'Product Unique Barcode',
    'version': '1.1',
    'author': 'Husam',
    'category': 'Sales Management',
    'depends': ['base', 'process', 'decimal_precision', 'mail'],
    'description': """
        Unique Barcode of Product 
    """,
    'data': [],
    'installable': True,
    'auto_install': False,     }

3) unique_product.py

from openerp.osv import fields,osv

class product_product(osv.osv):
    _inherit = 'product.product'
   _sql_constraints = [
        ('unique_product_barcode', 'unique(ean13)', 'Product Bar-Code  Must Be Unique .'),
     ]

product_product()

Restart Your OpenERP Go to Settings->Install Modules->find unique_product_barcode and install