avoid duplicated fields in read

Applies to: 7.0 and above

Steps to reproduce:
  1. add the field user_id to crm.lead tree view (so we have twice the field user_id)
  2. load the view

Result:
  If at least one lead is unassigned, there will get a traceback

When we were reading twice a m2o field where at least one result is null, the first call to name_get would set the value to False instead of None and then accepted by the filter 'isinstance(x, (int,long))'