Refactor: Check if both x and x_callback is given

[selmanozleyen]

When one gives both of them, one of them are overridden implicitly. We should throw an error when both is given.

[MUCDK]

Hi @selmanozleyen ,

What is the current state on this?

[selmanozleyen]

would be good to have but I think it's low-priority compared to other issues