Prevent callback from being set twice.

When you add one callack in two separate `set_callback` calls - it is
only called once.

When you do it in one `set_callback` call - it is called twice.

This violates the principle of least astonishment for me. Duplicating
callback is usually an error. There is a correct and obvious way to do
anything without this "feature".

If you want to do

    before_save :clear_balance, :calculate_tax, :clear_balance

or whatever, you should better do

    before_save :carefully_calculate_tax

    def carefully_calculate_tax
      clear_balance
      calculate_tax
      clear_balance
    end

And this even opens gates for some advanced refactorings, unlike the
first approach.

My assumptions are:

- Principle of least astonishment is violated, when callbacks are either
  prevented from duplication, or not.
- Duplicating callbacks is usually an error. When it is intentional -
  it's a smell of a bad design and can be approached without abusing
  this "feature".

My suggestion is: do not allow duplicating callbacks in one callback
call, like it is not allowed in separate callbacks call.

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