Asturian conmigo, Portuguese comigo, Galician comigo, Dalmatian conmaic
The word mecum in Latin became *mego/*migo in Vulgar Latin. The phonological change from -cum to -go led to confusion of its meaning, as it was no longer identifiable as the word cum "with." By the time of Old Spanish, the word (now evolved into migo) was unclear and con- was added to reinforce its meaning. The same process happened in contigo, consigo.