Sorry for the delay in replying, i didn't see this originally.
There is another settings in AS that could be causing this:
Try disabling that option.
Let me know if that's the cause -- I think I need to move this into capture rules area.
I was thinking I could also add something that kept a hash value for the last X screenshots, so it wasn't always just comparing to the LAST screenshot to see if its a duplicate but rather the last X screenshots..