What is the origin of "litera" versus "littera"?

Is there a history behind the dual spelling "littera" and "litera"?

This comes from a sporadic sound change in Latin, conveniently known as the littera rule. When a short voiceless stop (p, t, or c) comes after a long high vowel (ī or ū), it sometimes changes to a short vowel and a long consonant: lītera~littera, Jūpiter~Juppiter, *cīpus~cippus, mūcus~muccus, cūpa~cuppa, *sūpus~suppus, *mītō~mittō. But note the "sometimes": we see dūcō, but never **duccō, for example, and in many cases the original long-vowel form survived alongside the long-consonant form.

(When a form is marked with a star, that means that exact form isn't attested in writing, but must have existed etymologically. For example, if I'm reading Weiss correctly we don't see *cīpus anywhere, but we do see the Old Latin form ceipos, and the Classical form cippus, and we know that ei > ī and -os > -us during the evolution into Classical Latin. Two stars means that form doesn't exist, and probably never existed.)

See Weiss 2010 for a more detailed overview.