TDB detects attempt to reassign a variable.

PREFIX  :     <http://example/>

SELECT  *
WHERE
  { ?x :p 123
    { SELECT  ?x
      WHERE
        { ?x :p ?o }
    }
  }

20:24:32 ERROR QueryIteratorCloseable    :: QueryFatalException
com.hp.hpl.jena.sparql.ARQInternalErrorException: Attempt to reassign '?x' from '<http://example/x>' to '<http://example/x>'
	at com.hp.hpl.jena.sparql.engine.binding.BindingHashMap.checkAdd(BindingHashMap.java:111)
	at com.hp.hpl.jena.sparql.engine.binding.BindingHashMap.add(BindingHashMap.java:92)
	at com.hp.hpl.jena.sparql.engine.binding.BindingFactory.materialize(BindingFactory.java:60)
	at com.hp.hpl.jena.tdb.solver.QueryEngineTDB$QueryIteratorMaterializeBinding.moveToNextBinding(QueryEngineTDB.java:132)
	at com.hp.hpl.jena.sparql.engine.iterator.QueryIteratorBase.nextBinding(QueryIteratorBase.java:154)