The original meaning of ‘mano a mano’ (hand to hand) is permanently ingrained in my mind. Whenever I hear people using it to mean ‘man to man’ I wonder if they also say ‘womano a womano’.

The bastardized forms of ‘mano a mano’ also conjure up some interesting pictures in my mind. For example, when a commentator for a bicycle race says something like ‘[RiderName] and [RiderName] are going at it mano a mano’, I envision two riders pedaling with their hands.

The ‘head to head’ meaning baffles me too, especially when a perfectly good ‘foreign’ phrase already exists for ‘head to head’ in ‘tête-à-tête’. Just as ‘vis-à-vis’ is a perfectly good ‘foreign’ phrase for ‘face to face’. I guess it’s really just the ‘a’ in the ‘[word] a [word]’ phrase that sells the tickets; fill in the meaning of the other words any way you want.