MySQL has recently added a second entry to the JDBC 4 service provider file (META-INF/services/java.sql.Driver). When adding the driver as a deployment, both driver classes are recognized and two driver instances are created. When adding the driver as a "core module", an exception is thrown stating that the driver name is a duplicate. It seems to be trying to create two driver instances with the same name, one for each entry in the service provider configuration.

Here's the exception:
21:56:38,713 ERROR [org.jboss.as.controller.management-operation] (ServerService Thread Pool – 28) JBAS014612: Operation ("add") failed - address: ([
("subsystem" => "datasources"),
("jdbc-driver" => "mysql2")
]): org.jboss.msc.service.DuplicateServiceException: Service jboss.jdbc-driver.mysql2 is already registered
at org.jboss.msc.service.ServiceRegistrationImpl.setInstance(ServiceRegistrationImpl.java:158) [jboss-msc-1.2.0.Final.jar:1.2.0.Final]
at org.jboss.msc.service.ServiceControllerImpl.startInstallation(ServiceControllerImpl.java:235) [jboss-msc-1.2.0.Final.jar:1.2.0.Final]
at org.jboss.msc.service.ServiceContainerImpl.install(ServiceContainerImpl.java:767) [jboss-msc-1.2.0.Final.jar:1.2.0.Final]
at org.jboss.msc.service.ServiceTargetImpl.install(ServiceTargetImpl.java:223) [jboss-msc-1.2.0.Final.jar:1.2.0.Final]
at org.jboss.msc.service.ServiceControllerImpl$ChildServiceTarget.install(ServiceControllerImpl.java:2401) [jboss-msc-1.2.0.Final.jar:1.2.0.Final]
at org.jboss.msc.service.ServiceBuilderImpl.install(ServiceBuilderImpl.java:317) [jboss-msc-1.2.0.Final.jar:1.2.0.Final]
at org.jboss.as.controller.OperationContextImpl$ContextServiceBuilder.install(OperationContextImpl.java:1418) [wildfly-controller-8.0.0.Final.jar:8.0.0.Final]
at org.jboss.as.connector.subsystems.datasources.JdbcDriverAdd.startDriverServices(JdbcDriverAdd.java:160)
at org.jboss.as.connector.subsystems.datasources.JdbcDriverAdd.performRuntime(JdbcDriverAdd.java:118)
at org.jboss.as.controller.AbstractAddStepHandler$1.execute(AbstractAddStepHandler.java:75) [wildfly-controller-8.0.0.Final.jar:8.0.0.Final]
at org.jboss.as.controller.AbstractOperationContext.executeStep(AbstractOperationContext.java:591) [wildfly-controller-8.0.0.Final.jar:8.0.0.Final]
at org.jboss.as.controller.AbstractOperationContext.doCompleteStep(AbstractOperationContext.java:469) [wildfly-controller-8.0.0.Final.jar:8.0.0.Final]
at org.jboss.as.controller.AbstractOperationContext.completeStepInternal(AbstractOperationContext.java:273) [wildfly-controller-8.0.0.Final.jar:8.0.0.Final]
at org.jboss.as.controller.AbstractOperationContext.executeOperation(AbstractOperationContext.java:268) [wildfly-controller-8.0.0.Final.jar:8.0.0.Final]
at org.jboss.as.controller.ParallelBootOperationStepHandler$ParallelBootTask.run(ParallelBootOperationStepHandler.java:343) [wildfly-controller-8.0.0.Final.jar:8.0.0.Final]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145) [rt.jar:1.7.0_45]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615) [rt.jar:1.7.0_45]
at java.lang.Thread.run(Thread.java:744) [rt.jar:1.7.0_45]
at org.jboss.threads.JBossThread.run(JBossThread.java:122) [jboss-threads-2.1.1.Final.jar:2.1.1.Final]

I have confirmed that even after this, there is one driver instance. If the driver class is given explicitly, the exception is not thrown.