Is Zakat to be paid for the same money stored yearly or is to be paid once only?

I recently heared that Zakaat is to be paid for the stored money only once, if any money is added to the stored sum then Zakat is paid for the amount added and not for the amount for which Zakaat was paid last year. Actually I have been always paying Zakaat for my total stored money yearly and I don't know whether the idea I heared is correct. For example: if I had $ 1000 last year and payed $ 25 last year as Zakaat, and suppose the money became now $ 2000, shall I now pay $ 25 for the amount added or shall I pay $ 50 for the total ? Jazakumullahu Khayran