Well, what dictionary are you using? Dictionary.com says one "n" is incorrect...as does Babylon's dictionary. My mom's also, as it happens, a professional proofreader, and has ranted at least once on the word. I think any dictionary that lists it as acceptable is merely taking into the account that such a vast number of people misspell it.
Yes, that's roughly the same screen I got. You'll notice it's not using the word in the same sense at all. There's only a Hebrew translation and one from a Christian dictionary which refers to God, and not to a length of 1,000 years. So, when in doubt, I look to the person who does it for a living that I'm fortunate enough to know, as well as another online dictionary...and both say it's wrong.