**Apache Airflow version**: 2.0.0

**Environment**:

- **Cloud provider or hardware configuration**: Docker container
- **OS** (e.g. from /etc/os-release): Debian Stretch

**What happened**:

When working on some custom DAG override logic, I noticed that invoking `DAG.partial_subset` does not properly update the corresponding `_task_group.used_group_ids` on the returned subset DAG, such that adding back a task which was excluded during the `partial_subset` operation fails.

**What you expected to happen**:

Tasks that had already been added to the original DAG can be added again to the subset DAG returned by `DAG.partial_subset`

**How to reproduce it**:
Create any DAG with a single task called, e.g. `my-task`, then invoke `dag.partial_subset(['not-my-task'], False, False)`

Note that the returned subset DAG's `_task_group.used_group_ids` still contains `my-task` even though it was not included in the subset DAG itself

**Anything else we need to know**:

I was able to work around this by adding logic to update the new partial subset DAG's `_task_group.used_group_ids` manually, but this should really be done as part of the `DAG.partial_subset` logic
airflow/models/dag.py
raise DuplicateTaskIdFound(f"Task id '{task.task_id}' has already been added to the DAG")
        else:
            self.task_dict[task.task_id] = task
            task.dag = self
            
            self._task_group.used_group_ids.add(task.task_id)
        self.task_count = len(self.task_dict)
    def add_tasks(self, tasks):
        """
        Add a list of tasks to the DAG
        :param tasks: a lit of tasks you want to add
        :type tasks: list of tasks
        """
        for task in tasks:
            self.add_task(task)
    def run(
        self,
        start_date=None,
        end_date=None,
        mark_success=False,
        local=False,
        executor=None,
        donot_pickle=conf.getboolean('core', 'donot_pickle'),
        ignore_task_deps=False,
        ignore_first_depends_on_past=True,
        pool=None,
        delay_on_limit_secs=1.0,
        verbose=False,
        conf=None,
        rerun_failed_tasks=False,