**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
"""
    Create the list of edges needed to construct the Graph view.
    A special case is made if a TaskGroup is immediately upstream/downstream of another
    TaskGroup or task. Two dummy nodes named upstream_join_id and downstream_join_id are
    created for the TaskGroup. Instead of drawing an edge onto every task in the TaskGroup,
    all edges are directed onto the dummy nodes. This is to cut down the number of edges on
    the graph.
    For example: A DAG with TaskGroups group1 and group2:
        group1: task1, task2, task3
        group2: task4, task5, task6
    group2 is downstream of group1:
        group1 >> group2
    Edges to add (This avoids having to create edges between every task in group1 and group2):
        task1 >> downstream_join_id
        task2 >> downstream_join_id
        task3 >> downstream_join_id
        downstream_join_id >> upstream_join_id
        upstream_join_id >> task4
        upstream_join_id >> task5
        upstream_join_id >> task6
    """
    
    edges_to_add = set()
    
    edges_to_skip = set()
    task_group_map = dag.task_group.get_task_group_dict()
    def collect_edges(task_group):