Purity: Correcting Incorrect Funeral Rites
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 533:
If someone knows that the ghusl, shrouding, prayers, or burial of a deceased person has been performed incorrectly, he must perform that act again. However, if one merely supposes that it was performed incorrectly or doubts whether it was performed correctly or not, then it is not necessary for him to perform it again.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)