Asturian contigo, Portuguese contigo, Galician contigo
The word mecum in Latin became *tego/*tigo in late 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 tigo) was unclear and con- was added to reinforce its meaning. The same process happened in conmigo, consigo.