Has an AFL game ever ended 51-42?
Exactly once. On June 14, 1919, Richmond beat St Kilda 51-42, and no AFL game before or since has ended that way. When it happened it was a scorigami, the first 51-42 in league history.
That makes 51-42 the 3836th most common final score out of 6,607 that have ever occurred in AFL. One point away, 52-42 has happened 3 times and 51-43 has happened never.