Updating existing merged documents does not work properly

[anthony-gray]

Updating existing merged documents does not work properly. If you have a merged document with URIs 1.xml and 2.xml, then introduce 3.xml into the system that matches to 1.xml and 2.xml, and then rerun matching and merging for 1.xml, 2.xml, and 3.xml, one would expect 1 merged document of 1.xml, 2.xml, and 3.xml, but this is not the case.