**Impacted versions:**
saas-14

**Steps to reproduce:**
1. create a sale order with a new product named Sprint configured as service, Track Service "Create task and create hours" and link to Internal - GAP Analysis.
2. Confirm the sale order and review
3. Add a new line for sprint in the sale order and try to save the file.

**Current behavior:**
A Odoo Server Error (Validation Error) appears "Error, a partner cannot follow twice the same object"

![image](https://cloud.githubusercontent.com/assets/7593953/23186396/9f050718-f84c-11e6-95c0-af0104c9cdc9.png)

**Expected behavior:**
I want to be able to add more lines to my confirmed sale order

**Video/Screenshot link (optional):**
https://youtu.be/ApXlPrxVISE

I think this is a critical bug. We found out this bug Feb 12, so maybe a change before that date could introduce the bug. 

If if help, I repeat the same poc in a local instance and this is the error capture in my server log
```bash
sale.order(1,).write({
    u'order_line': [[4, 1, False], [0, False, {
        u'procurement_ids': [],
        u'qty_delivered': 0,
        u'product_id': 1,
        u'product_uom': 1,
        u'sequence': 11,
        u'customer_lead': 0,
        u'price_unit': 0,
        u'product_uom_qty': 1,
        u'discount': 0,
        u'state': u'draft',
        u'qty_delivered_updateable': True,
        u'analytic_tag_ids': [],
        u'invoice_status': u'no',
        u'tax_id': [[6, False, [2]]],
        u'layout_category_id': False,
        u'name': u'Sprint'}]]})

2017-02-20 20:05:09,736 1513 INFO openerp_test odoo.sql_db: bad query:
UPDATE "mail_followers"
SET "res_model"='project.task'
WHERE id IN (29, 30) 

ERROR: duplicate key value violates unique constraint "mail_followers_mail_followers_res_partner_res_model_id_uniq"
DETAIL:  Key (res_model, res_id, partner_id)=(project.task, 16, 3) already exists.
```

from odoo import api, fields, models
class Followers(models.Model):
    """ mail_followers holds the data related to the follow mechanism inside
    Odoo. Partners can choose to follow documents (records) of any kind
    that inherits from mail.thread. Following documents allow to receive
    notifications for new messages. A subscription is characterized by:
    :param: res_model: model of the followed objects
    :param: res_id: ID of resource (may be 0 for every objects)
    """
    _name = 'mail.followers'
    _rec_name = 'partner_id'
    _log_access = False
    _description = 'Document Followers'
    res_model_id = fields.Many2one(
        'ir.model', 'Related Document Model',