Saxon

I think this is probably related to bug #3889, which proved very disruptive. Perhaps the patch for that bug wasn't applied correctly on the 9.9 branch, or was inconsistent with some other change.

Current state: I've reproduced the problem, but walking it through in the debugger, I can't see where the excess namespace declaration is coming from. There seems to be a NamespaceReducer in the pipeline that is successfully eliminating duplicate declarations, but somehow this one slips through...

There is a rather remarkable error in NamespaceReducer.namespace(), which is present also in 9.8. The code:

       for (NamespaceBinding ns : namespaceBindings) {
            if (isNeeded(ns)) {
                addToStack(ns);
                countStack[depth - 1]++;
                nextReceiver.namespace(namespaceBindings, properties);
            }
        }

processes a set of namespace bindings. If any of the namespace bindings is needed, then it outputs all of them.

The ability to pass multiple namespaces in a single call of Receiver.namespace() was introduced in Saxon 9.8, it was found to give significant performance improvements when doing a deep-copy of a tree in which elements had many (like a dozen) in-scope namespaces.

Why this obvious error hasn't caused more problems is anyone's guess.

Confirmed that correcting this code fixes the problem. More regression testing is needed before closing the bug, however.