Apparent bug resulting in multiple `most_recent = true` rows

[Tabby]

Seems like it's occasionally possible for concurrent updates to leave a transitions table in a state where two identical transitions are added (differing only in modification time) but with different primary keys and with the most_recent column set to true for both.

Not really sure how this is possible :S

Slack thread where the issue has been seen a couple of times: https://gocardless.slack.com/archives/C02AU55J6R4/p1652169426426829?thread_ts=1650356440.633239&cid=C02AU55J6R4

Slack thread asking about this issue and potential causes/solutions before reporting this issue: https://gocardless.slack.com/archives/C031GKFNK6J/p1652280367772639

[dmitry]

Do you have unique index key for the most_recent?