avoid duplicated fields in read

_read_flat: remove duplicated fields in read call
get many2one: as False is instance of int, check the value of x first to avoid calling a name_get with a list of False

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))'