Handle spurious ReceiveScatterOutput callbacks (#2051)

This commit fixes #2003 by handling spurious, repeated callbacks of the
`receive_scatter_output` method of the `ReceiveScatterOutput` class. The
reason of multiple awakenings has not been investigated deeply, though.
In the future, a thorough examination of the `MultithreadedJobExecutor`
logic may be necessary.

---------

Co-authored-by: Michael R. Crusoe <1330696+mr-c@users.noreply.github.com>

Author: GlassOfWhiskey

cwltool/workflow_job.py

@@ -10,6 +10,7 @@
     MutableMapping,
     MutableSequence,
     Optional,
+    Set,
     Sized,
     Tuple,
     Union,
@@ -88,12 +89,17 @@ def __init__(
     ) -> None:
         """Initialize."""
         self.dest = dest
-        self.completed = 0
+        self._completed: Set[int] = set()
         self.processStatus = "success"
         self.total = total
         self.output_callback = output_callback
         self.steps: List[Optional[JobsGeneratorType]] = []
 
+    @property
+    def completed(self) -> int:
+        """The number of completed internal jobs."""
+        return len(self._completed)
+
     def receive_scatter_output(self, index: int, jobout: CWLObjectType, processStatus: str) -> None:
         """Record the results of a scatter operation."""
         for key, val in jobout.items():
@@ -108,10 +114,11 @@ def receive_scatter_output(self, index: int, jobout: CWLObjectType, processStatu
             if self.processStatus != "permanentFail":
                 self.processStatus = processStatus
 
-        self.completed += 1
+        if index not in self._completed:
+            self._completed.add(index)
 
-        if self.completed == self.total:
-            self.output_callback(self.dest, self.processStatus)
+            if self.completed == self.total:
+                self.output_callback(self.dest, self.processStatus)
 
     def setTotal(
         self,