Occasional UniqueViolation in device_lists_remote_cache_unique_id

[matrixbot]

This issue has been migrated from #16243.

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e.g.

• https://sentry.tools.element.io/organizations/element/issues/71346/
• https://sentry.tools.element.io/organizations/element/issues/78907/
• https://sentry.tools.element.io/organizations/element/issues/84351/

From looking at the source, the only insert that isn't an upsert I can see is

https://github.com/matrix-org/synapse/blob/69048f7b4848ab6a4ae6cb233f8cbf36d73c0ba1/synapse/storage/databases/main/devices.py#L1911-L1919

which might emit this error if

• the same (user, device) ID appears twice in this batch of queries, or
• this (user, device) pair already exists in the DB.

Tracebacks in Sentry seems to corroborate my suspicions --- they have the quoted function in their stack.