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.
addons/crm/res_partner.py
categ_obj = self.pool.get('crm.case.categ')
        categ_ids = categ_obj.search(cr, uid, [('object_id.model','=','crm.lead')])
        lead_obj = self.pool.get('crm.lead')
        opportunity_ids = {}
        for partner in self.browse(cr, uid, ids, context=context):
            if not partner_id:
                partner_id = partner.id
            opportunity_id = lead_obj.create(cr, uid, {
                'name' : opportunity_summary,
                'planned_revenue' : planned_revenue,
                'probability' : probability,
                'partner_id' : partner_id,
                'categ_ids' : categ_ids and categ_ids[0:1] or [],
                'type': 'opportunity'
            }, context=context)
            opportunity_ids[partner_id] = opportunity_id
        return opportunity_ids
    def schedule_meeting(self, cr, uid, ids, context=None):
        partner_ids = list(ids)
        partner_ids.append(self.pool.get('res.users').browse(cr, uid, uid).partner_id.id)
        res = self.pool.get('ir.actions.act_window').for_xml_id(cr, uid, 'calendar', 'action_calendar_event', context)
        res['context'] = {
            'search_default_partner_ids': list(ids),
            'default_partner_ids': partner_ids,
        }
        return res