Hi,
In addition to what Eliot said, for the Oxygen user's guide we also have
this rule of the topic ID equal to the file name.
We impose it with a Schematron check along with a quick fix to change
the ID if necessary, search for "Topic ID must be equal to file name":
As Radu points out, the DITA specification does not require that topic IDs be unique beyond the scope of the XML document that contains them. In general, especially when using keys and with the addition of the “same topic” fragment identifier (‘#./some-element-id’), topic IDs are of little
