releases sometimes end up with duplicate data versions

[bhearsum]

While working on a script to import releases_history from mysql -> GCS I discovered that many (most?) releases have duplicate data versions in their history. For example, Devedition-54.0b11-build3 has 480 entries in the history table (not including the initial NULL insert entry), but the maximum data version is 474.

data_version 12 is one of the duplicate ones. The first row with that data version adds the "he" locale. The second row for that data_version removes it and adds "sr" instead, and its timestamp is one second later. This leads me to think that the update race merging logic is at fault.

(Imported from https://bugzilla.mozilla.org/show_bug.cgi?id=1549367)