**Apache Airflow version**:
2.2.0.dev0 (possible older versions too)

**OS**:
Linux Debian 11

(BTW Here we have a typo in comment inside bug report template in `.github/ISSUE_TEMPLATE/bug_report.md`: `cat /etc/oss-release` <- double 's'.)

**Apache Airflow Provider versions**: -

**Deployment**:
Docker-compose 1.25.0

**What happened**:
The issue is related to Airflow UI and it shows duplicated notifications when removing all the tags from the input after filtering DAGs.

**What you expected to happen**:
The notifications should not be duplicated.

**How to reproduce it**:
In the main view, above the list of the DAGs (just below the top bar menu), there is a place where notifications appear. Suppose that there are 2 notifications (no matter which). Now try to search DAGs by tag using 'Filter DAGs by tag' input and use a valid tag. After the filtering is done, clear the input either by clicking on 'x' next to the tag or on the 'x' near the right side of the input. Notice that the notifications are duplicated and now you have 4 instead of 2 (each one is displayed twice). The input id is `s2id_autogen1`.
This bug happens only if all the tags are removed from the filtering input. If you remove the tag while there is still another one in the input, the bug will not appear. Also, it is not present while searching DAGs by name using the input 'Search DAGs'.

After search, before removing tags from input:
![notif](https://user-images.githubusercontent.com/7412964/130072185-ca2b3bd4-023d-4574-9d28-71061bf55db6.png)
Duplicated notifications after removing tag from input:
![notif_dup](https://user-images.githubusercontent.com/7412964/130072195-1d1e0eec-c10a-42d8-b75c-1904e05d44fc.png)

**Are you willing to submit a PR?**
I can try.
airflow/www/views.py
"""
        A restful endpoint that returns external links for a given Operator
        It queries the operator that sent the request for the links it wishes
        to provide for a given external link name.
        API: GET
        Args: dag_id: The id of the dag containing the task in question
              task_id: The id of the task in question
              execution_date: The date of execution of the task
              link_name: The name of the link reference to find the actual URL for
        Returns:
            200: {url: <url of link>, error: None} - returned when there was no problem
                finding the URL
            404: {url: None, error: <error message>} - returned when the operator does
                not return a URL
        """
        dag_id = request.args.get('dag_id')
        task_id = request.args.get('task_id')
        execution_date = request.args.get('execution_date')
        link_name = request.args.get('link_name')
        dttm = timezone.parse(execution_date)
        dag = current_app.dag_bag.get_dag(dag_id)
        if not dag or task_id not in dag.task_ids:
            response = jsonify(
                {