I've found this same issue and can be reproduced systematically. Just check out the Any23 library source trunk from google code ( svn checkout http://any23.googlecode.com/svn/trunk/ any23-read-only ), build it through Maven or Ant and run the test org.deri.any23.extractor.rdfa.RDFaExtractorTest with the assertions enabled.
You'll obtain this exception:
java.lang.AssertionError: Created a duplicate MemLiteral for literal "2004-12-20"^^<http://www.w3.org/2001/XMLSchema#dateTime>
at org.openrdf.sail.memory.model.MemValueFactory.createMemLiteral(MemValueFactory.java:328)
at org.openrdf.sail.memory.model.MemValueFactory.createMemValue(MemValueFactory.java:215)
This test aims to verify a tolerant parsing support, in fact as you can notice the literal is a date declared as a datetime.
Any23 is currently using Sesame 2.2.4, a version lover to the declared fix verions, but I tried also an update to 2.3.1 obtaining the same result.
The invalid dateTime value was stored internally as a xsd:date value. A second attempt to create the same value would fail because the MemoryStore couldn't find the first value, but after creating it detected that it created a duplicate.
Bug
Resolved
Major
Created a duplicate MemLiteral for literal "2004-12-20"^^<http://www.w3.org/2001/XMLSchema#dateTime>