By default JPA don't have this feature, also this issue is more related how to database manage the sequences and as far as I know sequence number are not reused.
Persist operation will create a new id, when the entity does not have one, if you try to persist a detached object probably you will find a EntityExistsException.
• If the Object is a removed entity instance, an IllegalArgumentException will be thrown by the merge operation (or the transaction commit will fail).