Redefine parameter in Monte Carlo analysis

Hi,

I am using GLPK to optimize an energy system model for Switzerland. Lots of parameters such as costs or efficiencies are defined in a dat file. Now I want to systematically vary some of these parameters using a Monte Carlo approach. The easiest way to do this would be to just create a small addendum to the dat file that redefines / overrides the parameter values that were previously defined in the dat file, and to paste these two together. Unfortunately, that doesn't work, the solver just complains that a parameter was already defined.

Any help is greatly appreciated.

Best regards,

Gianfranco