extended check for "XML-metadata validity"

[ptahmose]

Is your feature request related to a problem? Please describe.
Beyond conformity to schema, there are semantic checks on the data like "uniqueness of IDs" which should be done.

Describe the solution you'd like
Where the semantic requires unique Ids, it should be checked that the Id-attribute is unique. Here is an example of a malformed XML:

Describe alternatives you've considered
Expressing this condition in the XML-schema - not sure if that's possible.