Impacted versions: 8.0, 9.0, 10 (both enterprise and community)

Steps to reproduce:
- Create a new calendar item in the agenda and add yourself as attendee:
![image](https://cloud.githubusercontent.com/assets/6352350/26573142/e7d4b368-451c-11e7-921d-5e56a9e66066.png)
- Go to users and duplicate the user who you've just attached to the calendar item:
![image](https://cloud.githubusercontent.com/assets/6352350/26573178/071afa8e-451d-11e7-8c2e-9ac0b6085e0f.png)
Example of the new user:
![image](https://cloud.githubusercontent.com/assets/6352350/26573201/1fbbea8a-451d-11e7-906b-e47976ae6efb.png)
- Go back to the calendar and look at the attendees, the new (duplicated user) is automatically added as a follower:
![image](https://cloud.githubusercontent.com/assets/6352350/26573232/3eef796c-451d-11e7-8400-f84279486c0b.png)
![image](https://cloud.githubusercontent.com/assets/6352350/26573262/53590fbc-451d-11e7-8179-a76de4e3af7e.png)


Current behavior: When the user is duplicated the newly duplicated user will be added as a follower on all calendar items of the original user.

Expected behavior: The new, duplicated, user is not added as an attendee on all calendar items from the original user.
search_view = self.pool.get('ir.model.data').get_object_reference(cr, uid, 'base', 'view_res_partner_filter')
        value = {
            'domain': "[]",
            'view_type': 'form',
            'view_mode': 'form,tree',
            'res_model': 'res.partner',
            'res_id': int(partner_id),
            'view_id': False,
            'context': context,
            'type': 'ir.actions.act_window',
            'search_view_id': search_view and search_view[1] or False
        }
        return value
    def make_opportunity(self, cr, uid, ids, opportunity_summary, planned_revenue=0.0, probability=0.0, partner_id=None, context=None):