The sessions specification says that ClientSession should not attempt to implement any thread safety guarantees, and the Java driver does not do so. But the interpretation of that with respect to ClientSession#close leads to a race condition if multiple threads call the method concurrently on the same instance. This race can lead to duplicate server sessions added to the server session pool, which is undesirable.

The driver should protect against this in ClientSession#close by using proper thread synchronization (e.g. AtomicBoolean#compareAndSet.