The way Destination goals work is they get logged when ever anyone hits the URL. So if the person has been there before it will be logged a second time.
To answer your question its not unique and i don't think you can change that. Only way i can think of to maybe get around that would be to set a cookie on there browser and use that to change the URL adding something like ?hasbeenhere=1 and then set up a dynamic URL that would ignore it in the Goal. But i haven't tried that it was just an idea.
Dynamically generated or variable URLs
I hope this helps.