The English language is strange. For example, why can something be "undoubtable," and yet can't be "undoubtably" (as there's no such word) true ? We have to use "undoubtedly" - it's tricky, all too easy to slip up. I had to check my dictionary on that one, just to be sure.
Of course, to make matters even more complicated, there is also "indubitably."
I did a Google seach for "undoubtably" and got 448,000 links, although a handful of those were pointing out the misspelling.
