title: skip persisting entities if unique key matches
body: <p>Lets say thay I have an entity with name "A".It has two columns named as name and systemId.
  These columns constraint unique key.For example it does not allow duplicate entities with systemId "1" and name "test".</p>
  
  <p>My question is: I want to insert Collection of A entities.I want to skip duplicate entities just insert new ones.Is EntityManager able to skip these entities if unique constraint fails?
  How can I succeed this?</p>