If you write code that creates content nodes on document A: docA->CreateElementNS(..., &newElement); then appends them to document B: nodeInDocB->AppendChild(newElement, ..); then it's possible to get content node ID conflicts. It would seem that copying or moving nodes between documents should ensure that content IDs (gotten via GetContentID() remain unique for each document. If a node is in 2 documents at the same time, then, I guess, it would have to have an ID per document.
Content ID's are evil. If at all possible we should work on removing them in stead of making them...
Mass-reassigning bugs to firstname.lastname@example.org
Looks like they were removed at some point.