kobotoolbox / kpi / 25747273888

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fix(exports): prevent simultaneous identical exports DEV-2021 (#7028)

### 📣 Summary
Identical duplicate API export requests are now intercepted and
deduplicated. Instead of creating a new background Celery task, the API
queries for an identical running export task belonging to the same user
and returns it seamlessly to the frontend.

### 📖 Description
Previously, if the frontend (or a script) blindly retried this POST
request with the exact same JSON payload before the first export
finished, the system would make another background Celery task with the
same parameters. The new implementation within
`ExportTaskSerializer.create()` fixes this by first checking the
database for any active SubmissionExportTask belonging to the current
user that has an identical data payload (ensuring the configurations
like fields, group_sep, type, etc., match exactly). If a matching task
is found that is still in a CREATED or PROCESSING state, the serializer
bypasses database creation and returns the existing task. This allows
the frontend to continue polling the original task gracefully without
duplicating the task and wasting resources.


### 👀 Preview steps

1. ℹ️ have an account and a project
2. Simulate a long export by adding `time.sleep(30)` to the method
`SubmissionExportTask,_run_task` at
`/kpi/kpi/models/import_export_task.py`
3. Export the project data
4. within 30 seconds, duplicate your browser tab and export the data
again with the same export configuration
5. 🔴 [on main] notice that this creates a duplicated export task
6. 🟢 [on PR] notice that this doesn't create a duplicated export task

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