cheilanthes with two l's

The doubled l is a reaction to the stress, which falls on LAN and lands heavily enough that the consonant in front of it seems to be doing double duty. That instinct is sound for English words, where a doubled letter after a short vowel is normal. It does not apply here. The l belongs to cheilos, the lip, and there is only ever one of it.

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