Fix duplicated axiom spotted by D. Mentré.
Fix duplicated axiom spotted by D. Mentré.

ssaavedra/why3,jpdeplaix/why3,ssaavedra/why3,douglas-larocca/why3,douglas-larocca/why3,jpdeplaix/why3,jpdeplaix/why3,ssaavedra/why3,douglas-larocca/why3