Fixtures, uniqueness validation.

I'm watching the "Uniqueness Validation" video of the Building a Simple Rails Application series. I'm slightly confused as to how the uniqueness validation test actually proves anything.

The test only puts one name into the test database, and the validation validates that it is indeed unique. But it only puts one name in, so of course it's going to be unique?

Surely a valid test would be to put two entries into the test database with the same profile name, and then assert that user errors exist by running:

    assert !user.errors[:profile_name].empty?

at the end of the test?

I tried doing this test myself, but I can't figure out how to put two "users" into the users.yml fixture. But this,

jason:
 first_name: "Jason"
 last_name: "Seifer"
 profile_name: "Hiya"
 email: "jason@email.com"

jim:
 first_name: "Jim"
 last_name: "Jimson"
 profile_name: "Hiya"
 email: "jim@email.com"

results in a

ActiveRecord::Fixture::FormatError: a YAML error occurred parsing

error.