Alms Tax: Doubt Regarding the Payment of Zakat
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1992
If a person doubts whether or not he gave the zakat that was obligatory for him, and the property that was subject to zakat still exists, he must give zakat even if his doubt is about the zakat of previous years. However, if the property has perished, then zakat is not liable on it even if it relates to the current year.
-Ayatullah Sistani, Practical Laws of Islam, Alms Tax (Zakat)