Im amazed (and sschh I even googled it).That is the way I was taught how to spell it. Found I certainly wasnt alone -millions use the'n' and its obviously wrong.Wonder how that came about-really puzzled now.
There is a separate WORD, lemNa, in modern Latin, meaning a kind of aquatic plant sometimes confused with duckweed. It is listed in Linnæus's 'Genera Plantarum'.
However, as already made clear above, it has naught to do with dilemma!
I've never heard of dilemna. Perhaps it's one of those regional words. For example, in Bristol people say "I've got a good IDEAL", but they really mean IDEA. But to us, it has a different meaning. I original thought my Bristol friend was just being stupid, but then I heard all his friends saying the same thing too.
As you pointed out, Notafish, some Bristolians tend to put a letter 'l' after the final 'a' in words ending thus. Accordingly, for them, what the rest of us call a camera obscura is a cameral obscural. For information on what THAT is, click
http://en.wikipedia.org/wiki/Camera_obscura