Using microdata markup such as this on Dani's profile page with URL http://www.example.com/dani.html

<body itemscope itemtype="http://schema.org/ProfilePage">
  <h1 itemprop="name">About Dani</h1>
  <div itemscope itemprop="about" itemtype="http://schema.org/Person">
    <link itemprop="url" href="http://www.example.com/dani.html" />
  </div>
</body>

Is it appropriate to set the URL element to the ProfilePage item or the URL element for the Person item to http://www.example.com/dani.html? From what I understand, the URL is used as the unique identifier ID?

In the schema example they use an anchor tag instead of link, but otherwise it's the same.

... I've only used schema once and basically made a template page for the types so I never had to do them again...

I think it makes sense that the URL is a thing that belongs to your person...

Basically what confuses me is that, while the microdata markup I posted appears correct, both the ProfilePage item and the Person item both are associated with the url http://www.example.com/dani.html

Can multiple items have the same url?? I thought not ...