[Drunken F00l]

Something like?:

Code:

int isDuplicate = 0;
int newNumber = your_random_number_function();

// check for duplicate
for(int i = 0; i < 6; i++)
{
    if(b[i] == newNumber)
    {
        isDuplicate = 1;
    }
}

if(!isDuplicate)
{
    // add to array here
}