If a nodeset contains multiple elements with the same value then unioning
the nodeset with itself will not eradicate those duplicates.

If I have a nodeset containing

<a>1</a>
<a>2</a>

which I use as a lookup, then a nodeset created just by looking up the
value 1, 5 times will not contain <a>1</a> once and not 5 times.

The latter case is because a nodeset cannot contain multiple copies of the
same node.

Please correct my thinking here.