"We have already found evidence that he took eight jabs from four places," Amarendra Pratap Shahi, civil surgeon of Madhepura, told the BBC.
Early inquiries into the case indicated the senior citizen scored "two jabs in a half-hour gap" on the same day and each of these jabs "were registered on the portal".
"We are flummoxed how this could happen. There seems to be a portal failure happening. We are also trying to find out whether there was any negligence by people manning the vaccination centres," Shahi said.
The elderly man used different identity cards and mobile phone numbers of his relatives. He travelled to vaccination camps across Madhepura and two neighbouring districts to get vaccinated.
According to the Independent, Mandal first received his jab on February 13 last year then received a shot each in March, May, June, July and August.
In September, he managed to get three injections by using three different identity cards belonging to him. The IDs included his voter ID card and Aadhaar card, an ID that assigns a unique 12-digit identification number to each citizen.
By the end of 2021, the retiree claimed he had received 11 Covid vaccinations.
"After taking the jabs my body aches and pain disappeared. I used to have knee pain and walked with a stick. Now I don't. I feel fine," he told the BBC.
The story comes as India races to get inoculated across the country.
Prime Minister Narendra Modi said the government would vaccinate all 1.3 billion of the country's residents by December 31 last year.
To date, about 65 per cent of India's adult population is fully vaccinated while about 91 per cent have received at least one dose.