How do I test that a method was called within a grails asynchronous promise?

I'm trying the verify that a method contained in a promise was called, using a unit test.

I have a CallerService, that will call classA.methodA() within a task,

class CallerService {

def classA

 def callA() {
      Promise p  = task {
         classA.methodA()invocation)
     }
     p.onComplete {
         println "complete"
     }
     p.onError { Throwable err ->
         println "there was an error"
     }
 }
}

and a unit test that mocks ClassA and tries the verify that methodA was called once.

def "calling A"() {
    given:"a mock classA"
    def mockClassA = Mock(ClassA)
    service.classA = mockClassA

    when:"callA is called"
    service.callA()

    then:"methodA should be called once"
    1* mockClassA.methodA()
}

The tests fails because the mock was called twice.

| Failure:  calling A(promisespike.CallerServiceSpec)
|  Too many invocations for:
1* mockClassA.methodA()   (2 invocations)
Matching invocations (ordered by last occurrence):
2 * mockClassA.methodA()   <-- this triggered the error

Is this the result I should be expecting or have I setup my test incorrectly?