The regression has been introduced by JIRA https://issues.jboss.org/browse/RESTEASY-1284.
JAX-RS components are now registered only once but calling "getConfiguration().getClasses()" on the JAX-RS configurable instance (WebTarget for example) returns the set of registered JAX-RS component classes to be instantiated and
also the classes of registered singletons.
According to the documentation it should only return "the immutable set of registered JAX-RS component (such as provider or feature) classes to be instantiated, injected and utilized in the scope of the configurable instance.", not the classes of singletons to be utilized. (see attached test file)