duplicate sequence, it is possible?

Hi everyone.

I have a table with a PK, this table has a lot of insert per second (100 ~ 150 insert /sec)

Sometimes, a get a duplicate key error, but ID is generated from a function (VOLATILE). This function has just one line (select nextval('XXX'))

Is possible that this function return same ID for two diff call?

Postgres version is 9.0

Thanks!