Fix problem related to duplicate beans

This check is safe to have in place and makes the problem go away but
there is a root problem that the artifact is being mocked multiple
times because of the way _artefactInstance is managed in
ParameterizedGrailsUnitTest.

Related to #18

apps/demo33/grails-app/services/demo/PersonService.groovy

@@ -0,0 +1,8 @@
+package demo
+
+import grails.gorm.services.Service
+
+@Service(Person)
+abstract class PersonService {
+    abstract List<Person> list()
+}

apps/demo33/src/test/groovy/demo/PersonServiceSpec.groovy

@@ -0,0 +1,24 @@
+package demo
+
+import grails.testing.gorm.DataTest
+import grails.testing.services.ServiceUnitTest
+import spock.lang.Issue
+import spock.lang.Specification
+
+@Issue('grails/grails-testing-support#18')
+class PersonServiceSpec extends Specification implements ServiceUnitTest<PersonService>, DataTest {
+
+    void setupSpec() {
+        mockDomain Person
+    }
+
+    void 'test one'() {
+        expect:
+        service.list().size() == 0
+    }
+
+    void 'test two'() {
+        expect:
+        service.list().size() == 0
+    }
+}

grails-gorm-testing-support/src/main/groovy/grails/testing/gorm/DataTest.groovy

@@ -36,6 +36,7 @@ import org.grails.testing.GrailsUnitTest
 import org.grails.testing.gorm.MockCascadingDomainClassValidator
 import org.grails.testing.gorm.spock.DataTestSetupSpecInterceptor
 import org.grails.validation.ConstraintEvalUtils
+import org.springframework.beans.factory.support.BeanDefinitionRegistry
 import org.springframework.context.ConfigurableApplicationContext
 import org.springframework.transaction.PlatformTransactionManager
 import org.springframework.validation.Validator
@@ -97,9 +98,11 @@ trait DataTest extends GrailsUnitTest {
     void mockDataService(Class<?> serviceClass) {
         Service service = (Service)dataStore.getService(serviceClass)
         String serviceName = Introspector.decapitalize(serviceClass.simpleName)
-        applicationContext.beanFactory.autowireBean(service)
-        service.setDatastore(dataStore)
-        applicationContext.beanFactory.registerSingleton(serviceName, service)
+        if(!applicationContext.containsBean(serviceName)) {
+            applicationContext.beanFactory.autowireBean(service)
+            service.setDatastore(dataStore)
+            applicationContext.beanFactory.registerSingleton(serviceName, service)
+        }
     }
 
     AbstractDatastore getDataStore() {