Airflow does not warn if we create 2 dags with the same dag_id

[dimberman]

Apache Airflow version:

Kubernetes version (if you are using kubernetes) (use kubectl version):

Environment:

• Cloud provider or hardware configuration:
• OS (e.g. from /etc/os-release):
• Kernel (e.g. uname -a):
• Install tools:
• Others:
  What happened:

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What you expected to happen:

How to reproduce it:

Anything else we need to know:

Moved here from https://issues.apache.org/jira/browse/AIRFLOW-377