The tdml.xsd has uniqueness constraints for the test case names.
However, looking at the TestDFDLExpressions.scala, which uses Functions.tdml, if you modify to turn on validation of the tdml file, it reports no problems.
Yet there are a bunch of duplicates in Functions.tdml (see DFDL-1298).
So perhaps some validator option has to be turned on (full schema checking ??) for Xerces to actually check these? Or perhaps those key constraints are simply not correctly expressed and so they aren't enforcing anything.