Purity: Doubts Regarding Performance and Completeness of Ghusl
Practical Laws of Islam as per the teachings of Ayatullah Sistani
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.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)