We discovered this issue by accident with our integration code. If two separate calls are made to CreateArr at the same time, containing the same records, so that they are processing simultaneously, the validation triggers do not catch duplicate records.
So for example, we have an object called "checkbook". We uploaded a record to "checkbook" from two separate sessions at the same time, with the same "Checkbook Id" value. There is a unique validation trigger that prevents multiple records with the same "Checkbook Id" value from being created. However, when the two CreateArr calls finish, both records are created successfully with the duplicated id.
This leads further to an ironic situation, where you can't update the duplicate records either, because you get the unique validation constraint error on edit.