Determine cause of duplicated highlights in the DB

https://sentry.io/hypothesis/h/issues/846053583/events/45521568081/

In all the cases I've looked at so far it appears that the exact same annotation occurs twice in the database. This suggests that for some reason the annotation is being created two times. Further debug is needed to determine why. Is this a lag in submission where the user is able to submit the same annotation multiple times? Is this a frontend bug of some sort? Further investigation is needed.

Below is a correlation between the throughput to the annotation create endpoint and the ConcurrentUpdate errors. Note where there is a spike in errors there is also a spike in the amount of annotations created thus it appears that ConcurrentUpdate errors are related to duplicated annotations being created.

Below are several snip-its of annotations inside the db. Note the duplication (the time stamp and id are the only thing shown here for privacy reasons).

Here's a link to the first occurrence of this problem reflected in duplicated annotations being created inside the db:
https://metabase.hypothes.is/question#eyJuYW1lIjpudWxsLCJkYXRhc2V0X3F1ZXJ5Ijp7InF1ZXJ5Ijp7InNvdXJjZS10YWJsZSI6NDcsImZpbHRlciI6WyJhbmQiLFsiPSIsWyJmaWVsZC1pZCIsNDAyMl0sNTA1MjQ2XV19LCJ0eXBlIjoicXVlcnkiLCJkYXRhYmFzZSI6NX0sImRpc3BsYXkiOiJ0YWJsZSIsInZpc3VhbGl6YXRpb25fc2V0dGluZ3MiOnt9fQ==

Start with the linked Sentry event and compare the annotation create endpoint throughput with the ConcurrentUpdate errors. Inspect the duplicated annotation examples and determine whether repeated submission or a frontend issue explains the database duplicates; document the confirmed cause and supporting evidence.