sellaginella

This spelling is a reaction to the shape of the word rather than to any sound in it. Once a writer has committed to a doubled l at the end, a second one near the front starts to look like it belongs, and sell is a familiar English opening in a way that sel is not. The first syllable is bare: one s, one e, one l, and no weight on it at all.

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