Ref T13581. If you query for revisions by hash and provide multiple hashes (A, B) which match a single revision (e.g., older and newer diffs for that revision), the query omits a GROUP BY clause but should contain one.

Add a GROUP BY clause in this case.

With a working copy that has multiple hashes corresponding to a single revision, ran arc branches before and after the change. Before, got this error:

[2020-09-15 17:02:07] EXCEPTION: (ConduitClientException) ERR-CONDUIT-CORE: Rows passed to "loadAllFromArray(...)" include two or more rows with the same ID ("130"). Rows must have unique IDs. An underlying query may be missing a GROUP BY. at [<arcanist>/src/conduit/ConduitFuture.php:65]

After, clean execution.