Is @@IDENTITY Really Unique In Multi-sessions Scope

I have a table EugeneTest(id_num, fname, minit, lname)
where field "id_num" is type IDENTITY and has UNIQUE constraint

Let's say 2 user execute this code at the same time:

DECLARE @return integer
use EugeneTest
INSERT employees
( fname, minit, lname)
VALUES
( 'Eugene3', 'F', 'Josephs')
SET @return = @@IDENTITY

Is is not possible to get duplicated value in id_num column becuase of
UNIQUE
constraint, but is it possible the both user get the same @@IDENTITY
number
( for example, if @@IDENTITY is evaluated and kept somewhere in the
buffer before the data actually get written to the disk )

Thanks, Eugene