functions

Question

HI 

 public static void fun(double[] ff, double[] f, int []index, out double[] ff)
        {
            
            
                int i,edof,ii;
                edof = index.Length;
                
                for (i = 0; i < edof; i++)
                {
                    ii = index[i];
                    ff[ii] = ff[ii] + f[i];   
                }
         }

i have a function like this,

and the call as follows

 fun(ff, R3, ind1,out ff);

if i did like this ff is duplicate error,

how to i use the same ff array for input and output

please help in advance