@Override
    public void startPrefixMapping(final String prefix, final String uri) throws SAXException {
        if (depth > 1) {
            // If we are deeper than the root element then treat prefix mappings
            // as normal.
            super.startPrefixMapping(prefix, uri);

        } else if (root == null) {
            // Store prefix mappings and output them if we haven't seen the root
            // element yet.
            prefixDeque.push(new StartPrefixMapping(prefix, uri));
            super.startPrefixMapping(prefix, uri);
        }
    }
We want to suppress prefix mappings for first level elements that we see
again.
stroom-pipeline/src/main/java/stroom/pipeline/filter/MergeFilter.java