[takayoshi-makabe]

This is a follow-up to #68678, which proposed raising an ImportError when the same dag_id is found in two different files.

Currently, when two files define the same dag_id, Airflow silently overwrites the existing Dag entry with whichever file is parsed last — no error, no warning, no trace in the UI. This means a mis-deployed file can invisibly destroy a production Dag, leaving operators with no indication of what happened or where the conflict originated.

@uranusjr pointed out that an ImportError would fire transiently on every legitimate file rename, since the old DB entry persists until stale_dag_threshold passes. This PR avoids that entirely by recording a DagWarning instead of blocking the parse.

The warning shows both file names and the dag_id so users can self-diagnose:

dag_id 'hoge' is now served from 'file_b.py' (bundle: 'default'), previously registered from 'file_a.py'
(bundle: 'default'). If 'file_a.py' was renamed or moved, this notice will clear on the next parse
cycle once the old file is no longer observed. If both files coexist with the same dag_id, rename one of
them to avoid non-deterministic behavior.

For file moves the warning is transient and self-clearing; for real duplicates it persists until resolved.

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Was generative AI tooling used to co-author this PR?

• Yes — Claude Code (Sonnet 4.6)

[takayoshi-makabe]

The CI failure was in CI image checks / Static checks — the check-metrics-synced-with-registry hook requires every metric used in code to be registered in the metrics registry YAML, but dag_processing.duplicate_dag_id was missing.

Fixed by adding the metric to
shared/observability/src/airflow_shared/observability/metrics/metrics_template.yaml.

fed0d5e

[takayoshi-makabe]

Hi @potiuk,
I have fixed the issue that caused the previous CI failure. Could you please approve and trigger the tests again when you have a moment? Thank you!

[takayoshi-makabe]

@uranusjr
Thank you for the review. I've made some revisions based on your comments.

[takayoshi-makabe]

@uranusjr
Thank you for the approval. Could you please trigger the CI and merge the changes?