Handle duplicate functions in solution

[alexmojaki]

Sometimes students define the required function twice, e.g:

def double(x):
    return x + x

def quadruple(x):
    return double(double(x))

def quadruple(x):
    return double(double(x))

This causes an error in the checker making it seem like something has gone wrong. Instead we should just tell the user to define the function only once.