On pg. 410 I mention that Aulus Gellius (6.9) attests the form spepondī as an archaic but still classical variant for the regular spopondī. I should also have noted that the e-reduplicated form is also attested inscriptionally in the form SPEPODI in the wax tablets discovered in Moregine near Pompeii and composed by C. Novius Eunus (1st cent. CE, 15.2.11, 16.3.3, 17.3.5, 18.3.7). Interestingly some of these documents (15 and 18) exist in two versions, one by Eunus himself with many spelling "errors", and one by a professional scribe with correct spellings. In the case of this particular word the scribe uses the standard form spopondi at 15.5.9, 18.5.16.
See Adams, J. N. 1990. The Latinity of C. Novius Eunus. Zeitschrift für Papyrologie und Epigraphik 82:227–247.