If someone pays the owner of a public bath from money on which the one-fifth tax (khums) has not been paid, then although he has committed an unlawful act, the apparent ruling is that his ghusl is valid but he remains indebted to those entitled (mustaḥiqqūn) to receive khums. Ruling 383.* Someone who doubts whether or not he has performed ghusl must perform it.
However, if after performing ghusl, when the ghusl would commonly be considered finished, one doubts whether or not part of his head and neck or body has been washed, then in case he habitually performs the acts of ghusl in close succession and knows that he has washed most parts of his body, he must not heed his doubt.