Somebody just sent me a note that begins, "I don’t doubt that the letter is fake…".
From the context, I'm sure that the person who wrote the message to me is of the opinion that the letter is NOT fake. Perhaps he is using the word "doubt" in the sense of "suspect". Chinese do that all the time when they are thinking of huáiyí 懷疑, which means both "doubt" and "suspect", and then writing or speaking in English. Only rarely do I encounter a speaker / writer of English who confuses "doubt" and "suspect", as seems to be the case in this instance. However, since "doubt" means "disbelieve", misnegation may also be at issue here.