**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
from airflow.jobs.base_job import BaseJob
from airflow.jobs.scheduler_job import SchedulerJob
from airflow.jobs.triggerer_job import TriggererJob
from airflow.models import DAG, Connection, DagModel, DagTag, Log, SlaMiss, TaskFail, XCom, errors
from airflow.models.baseoperator import BaseOperator
from airflow.models.dagcode import DagCode
from airflow.models.dagrun import DagRun, DagRunType
from airflow.models.serialized_dag import SerializedDagModel
from airflow.models.taskinstance import TaskInstance
from airflow.providers_manager import ProvidersManager
from airflow.security import permissions
from airflow.ti_deps.dep_context import DepContext
from airflow.ti_deps.dependencies_deps import RUNNING_DEPS, SCHEDULER_QUEUED_DEPS
from airflow.utils import json as utils_json, timezone, yaml
from airflow.utils.dates import infer_time_unit, scale_time_units
from airflow.utils.docs import get_doc_url_for_provider, get_docs_url
from airflow.utils.helpers import alchemy_to_dict
from airflow.utils.log import secrets_masker
from airflow.utils.log.log_reader import TaskLogReader
from airflow.utils.session import create_session, provide_session
from airflow.utils.state import State
from airflow.utils.strings import to_boolean
from airflow.version import version
from airflow.www import auth, utils as wwwutils
from airflow.www.decorators import action_logging, gzipped
from airflow.www.forms import (
    ConnectionForm,
    DagRunEditForm,
    DateTimeForm,
    DateTimeWithNumRunsForm,