It has happened a few times back in 2004 (in edits 752619, 835828, 854892, 916266, 1136298, 1194378) that the same user cast two votes in an edit without the previous vote getting marked as superseded. We should

• fix those six cases (by marking the earlier vote as superseded),
• change the existing partial index ON vote (editor, edit) WHERE superseded = FALSE into a unique index to make sure that this can't happen again even when we slip a bug into the voting code.