Nicor Gas

Nicor wants me to pay for tenants gas... Tenant wasnt paying their gas bill in their name so nicor sent me the bill without any warning.. $700 tenant still lives in the house and is having a hard time paying nicor. I have contract saying tenant is responsible for gas in their name. Which gas bill is in their name. But nicor send me the bill now with my name on it and sent it to my personal home address, the tenant has a bill from nicor as well with his name on it... Nicor is trying to double collect this is way out of line there has to be a law they can send bills out to multiple people to see who pays it because what if I did pay it and tenant paid it... Nicor would have $700.00