This is a persistent problem in all classes. The Footys have specifically mentioned certain numbers in their class rules. The ones that read the same forward or backward. I've still seen #10 sailed at regattas though.
G.2.(c) boats with sail numbers 01,08,10,18,80 and 81 shall display a prefix 1, unless the number requires changing at an event due to a conflict. (http://footy.rcsailing.net/footyrules_2009.pdf
The RG65 Class just revised their sail marking rules too. They tackled the size and placement of jib numbers but not the content. Large size was deemed more important than uniqueness for jibs. RG65s struggle with fitting 2 digits on the main, my are they narrow.
The best solution I've seen was the addition of an X above the number. It was easy to see and understand. Everyone was notified during the skippers meeting too. 47 and 047 worked partly last weekend. I don't remember if the fleet was notified of the confusing pair, it may have helped.
I would certainly welcome a better solution.
(the guy who kept wondering if Scott was 80 or 08 until I read his main.)