SxS Header tweak

Probably easiest to explain by example. Previously, if we saw
```
<HD>Referring to 23(a)</HD>
<P>Content</P>
<HD>Referring to 23(a) as well</HD>
<P>Content</P>
```
we would have created two nodes associated with 23(a). When a user tried to get this SxS, they'd only see the first one (as it was the only that could be dereferenced). This patch makes the code keep track of the label it is processing, and, if that label comes up again, do not create a new labelled node.

In other words, 23(a) will have only one associated node and it will include both headers and both paragraphs.