'Operation with the same signiature as "withdrawAmount" already exists'

I did create a few Classes wich inherents from a base class.
Because I have to override some methodes I did create those methodes also in the subclass. When I do this I get the following error
’Operation with the same signiature as “withdrawAmount” already exists’

Why is this happen?
I did create a sample and include this with this email.

The strange thing is that I did create something similair with just methodes without parameters, then it works.
So I am lost here.

I hope someone can help me out.

Ai, when I did a preview, my attachment did get removed.
So here it is ( I hope ).