The calculation of the dication CO22+ density in the atmosphere of Mars is performed for the first time. The metastable lifetime of these species reaches 4 seconds. The density of a layer centered around 155--160 km altitude can reach from 3 to and 5 106 m-3. The ions are produced by CO2 photoionization and photoelectron impact on CO2. They are lost by dissociative recombination with the thermal electrons and chemical reaction with CO2. This work is based upon relevant chemical reaction coefficient rates that have been measured in laboratory very recently. We suggest that this ion layer can be detectable by a mass spectrometer onboard the orbiter of the CNES PREMIER 07 mission. This work opens a series of promising studies on double ionization processes in the Mars ionosphere, such as their implications for the production of energetic electrons or fast fragmentation ion products which could participate in atmospheric escape.