selaginnella and sellaginella, one doubled letter too many

Both of these spellings are the same instinct misfiring in different places. A word this long with a heavy stress near the end feels as though it should be doubling something on the way there, and writers try the l first, then the g, then the n. The doubling is real but it arrives later than any of them, in the suffix -ella, and it is the only pair in the word.

A consonant doubled to mark a stress, the way English words do it.