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.
res = {}
        for partner in self.browse(cr, uid, ids, context):
            res[partner.id] = len(partner.phonecall_ids)
        return res
    _columns = {
        'section_id': fields.many2one('crm.case.section', 'Sales Team'),
        'opportunity_ids': fields.one2many('crm.lead', 'partner_id',\
            'Leads and Opportunities', domain=[('probability', 'not in', ['0', '100'])]),
        'meeting_ids': fields.many2many('calendar.event', 'calendar_event_res_partner_rel','res_partner_id', 'calendar_event_id',
            'Meetings'),
        'phonecall_ids': fields.one2many('crm.phonecall', 'partner_id',\
            'Phonecalls'),
        'opportunity_count': fields.function(_opportunity_meeting_phonecall_count, string="Opportunity", type='integer', multi='opp_meet'),
        'meeting_count': fields.function(_opportunity_meeting_phonecall_count, string="# Meetings", type='integer', multi='opp_meet'),
        'phonecall_count': fields.function(_phonecall_count, string="Phonecalls", type="integer"),
    }
    def redirect_partner_form(self, cr, uid, partner_id, context=None):