I've seen a list someone made of all the examples of this in the Obama birth certificate. Two things stood out to me: first, they show up with about the same frequency as other examples I've found on the web (which are very easy to find); and second, cloned letters often turn up in parts of the document where it wouldn't make sense if someone was forging the document, like random letters in parts that are supposed to be part of the original form (not typed in).
The main thing that doesn't make sense to me about the cloned letter evidence is that I'd have to believe the forger cloned (copied and pasted) two of the boxes but not the third. Or, to use one of the other examples, the 'A' in one word and the first 'A' but not the second in another, in "STATE OF HAWAII."