Yes, this will always set the value of the first node. You'll need to select the list of @Q8OthIncYesNo2a nodes and set the value for each of them.
For example (note, I'm just 'freehanding' this and could easily have compilation errors, but it will give you the idea):
IXMLDOMNodeList nodes = thisXDocument.DOM.selectNodes("/dfs:myFields/dfs:dataFields/d:ADHS3243/@Q8OthIncYesNo1a");
foreach (IXMLDOMNode node in nodes)
{
node.text = 'n';
}