ICICI Bank — Unable to update car registration number on icici fastag

My car number is being used by another fasttag user. ICICI has issued a fasttag to another user who has registered it with my car number. There is no RC submitted for that fasttag. My car fasttag was registered using chassis number. When I tried to update my car registration number, it says it is already registered to another user. Called customer care and it says that there is a fasttag already issued on my car number, but it is being used by another person for the last 1 year. It is irresponsible on the part of ICICI Bank to issue a fasttag without a RC document verification. I want to update my car registration number on my fasttag. Help to get it updated.