One tenant over paid the other underpaid

The tenant in the lower unit is responsible for 1/2 of the electric bill and 1/2 of the gas bill. The tenant in the upper unit pays the other half. The lower tenant was paying the FULL electric bill and half of the gas bill for the last three years. She now claims that she didn't know she was only to pay HALF of the electric bill and wants the upper tenant to pay her back for the last three years. The lower tenant did sign a lease saying that she was responsible for only HALF of the bill. Can the lower tenant go after the upper tenant for the money? If it says in the lower tenants signed lease the she was only the pay half does that mean that even though she paid the full amount, that was her decision and she wouldn't be able to get the money back because she never asked for the upper unit the pay their half. The lower unit does get the eletric bill that needs to be split sent to her unit.

[Law200]

Normally the landlord would have to do that. You don't have an agreement with the tenant.

Technically yes you do need to repay if you agree it is an error; you can however argue hardship in having to pay all or any substantial portion--saying that it was their mistake after all, and you spent it w/o realizing it was in error, and paying back now would be difficult...they often will not sue or take any further court action in these cases but will try to work something out. They may even settle on a partial amount.