equissetum and equisettum are both out there

The two spellings put the doubled letter on either side of the same stressed vowel, one before and one after, which shows how little the ear can tell about where the weight is landing. English heard the emphasis on SEE and reached for a doubled consonant to mark it, as it would in bitter, letter or setting. There is nowhere in this word for that to happen. Both halves are ordinary Latin vocabulary with single consonants throughout.

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