Heres one of my A. heckelii with that double spot. (Excuse the crappy cell-phone shot I just grabbed).
3 of my 4 specimens have a second spot. 1 on one side only, another on both sides, but not bright, and this one, both are distinctly present on both sides.
As Japes mentioned though, I guess its common. Just figured I'd toss in an example of that spotting on A. heckelii.