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.
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from openerp.osv import fields,osv
class res_partner(osv.osv):
    """ Inherits partner and adds CRM information in the partner form """
    _inherit = 'res.partner'
    def _opportunity_meeting_phonecall_count(self, cr, uid, ids, field_name, arg, context=None):
        res = dict(map(lambda x: (x,{'opportunity_count': 0, 'meeting_count': 0}), ids))
        
        try:
            for partner in self.browse(cr, uid, ids, context):
                if partner.is_company:
                    operator = 'child_of'
                else:
                    operator = '='
                opp_ids = self.pool['crm.lead'].search(cr, uid, [('partner_id', operator, partner.id), ('type', '=', 'opportunity'), ('probability', '<', '100')], context=context)
                res[partner.id] = {
                    'opportunity_count': len(opp_ids),
                    'meeting_count': len(partner.meeting_ids),
                }
        except:
            pass
        return res
    def _phonecall_count(self, cr, uid, ids, field_name, arg, context=None):