Repetition within the Indus script always has a purpose. In this instance, two water symbols represent a large volume of water. In other words, Mahadevan's sign number 133 is an ideograph for deep water. Hence, it is the noun flood.
Illustrative Text Reference:
Mohenjo-daro: Seal: M-751 a: Sayid Ghulam Mustafa Shah and Asko Parpola, 1991: Corpus of Indus Seals and Inscriptions: Volume 2: Page 57: Collections in Pakistan: Suomalainen Tiedeakatemia.
Comment:
There is only one extant example of this sign in the corpus of Indus inscriptions.
Image Credit:
Indus Script Sign Number 133: Sign List of the Indus Script: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.