TMSS-610: Update observed_duration

SAS/TMSS/backend/src/tmss/tmssapp/models/specification.py

@@ -592,8 +592,18 @@ class SchedulingUnitBlueprint(RefreshFromDbInvalidatesCachedPropertiesMixin, Tem
         """
         return the overall observed duration of all (observation) tasks of this scheduling unit
         """
-        # TODO: Trigger an exception if a task overlaps with another. Just assume that they run subsequently for now.
         if self.observed_start_time and self.observed_end_time:
+            observed_tasks = []
+            for task in self.task_blueprints.all():
+                if task.specifications_template.type.value == TaskType.Choices.OBSERVATION.value and \
+                        (task.status == "observed" or task.status == "finished"):
+                    observed_tasks.append(task)
+            # TODO: For now we just assume that tasks run subsequently. Handle overlapping in future.
+            # Check if there are any overlapping obs task
+            for i, t1 in enumerate(observed_tasks):
+                for t2 in observed_tasks[i + 1:]:
+                    if t1.start_time < t2.start_time < t1.stop_time:    # Trigger an exception if any overlaps.
+                        raise Exception('There are at least two tasks overlapping: observed_duration is not reliable.')
             return self.observed_end_time - self.observed_start_time
         else:
             return None