XPATH Query String - Finding Duplicate Nodes

[pg_11]

The xml looks like this
<users>
<user><username>User1</username></user>
<user><username>User2</username></user>
<user><username>User3</username></user>
<user><username>User4</username></user>
<user><username>User1</username></user>
<user><username>User2</username></user>
<user><username>User5</username></user>
</users>

I am trying for a xpath query that would tell me whether there are any
duplicate user\username node in the xml. I dont event want what are
they. I just need whether there are any duplicates.
Can any one help me please