What is the ruling on doubting the payment of zakat on money after a long time has passed, and is it obligatory to pay it again?

We advise you to turn away from thinking about that, lest you open the doors of whispers (waswasa) upon yourself. As long as you have exerted your utmost effort to remember that and were unable, then there is no blame upon you, for Allah does not burden a soul beyond its capacity. The original state is that your conscience is clear until proven otherwise.