When domain model contains even one lazy reference to an object, default equals fails when it's compared to a) the actual implementation returned by Session.get/load or b) other proxies (at least of different supertype). Overriding equals on a class that uses surrogate id is not that simple. However, there is a simple solution to this problem:
In domain class, override equals like this:
This should result always in comparing object references of actual instances and thus preserve symmetry.
It's understandable that you don't wan to publish that kind of getImplementation utility e.g. in Hibernate, but maybe you could support this more directly by implementing
Hibernate.equals(Object o1, Object o2)