Which letters are never doubled in English words?

There is now a handy website for searching an English dictionary using regular expressions. It appears that the only letters that are never doubled are 'j', 'q', and 'x'. Mind you, you have to go to Uzbekistan to get their unit of currency which is named a "tyyn".

Perhaps I should make up a word such as "sajjaqquixx" to fix the missing ones. I'm not sure what it means yet, and neither is Google. Suggestions welcome.

Happy Christmas,

~Matt

Testing, testing

This log post should also appear on my Facebook page, along with a picture of our house as it was in 1948.