Am I completely liable for the difference when a roommate overpays me for rent?

Here is the living situation:

I live in a flat with 6 other people and am the one responsible for turning in rent and paying bills. I basically use a spreadsheet to calculate each person’s individual amount for bills and everyone adds that number to their individual rent amount. I am very communicative about the numbers whenever they change and send out monthly emails relating to these sorts of house-related things. Sometimes rent amounts will fluctuate due to long term guests, but everyone receives notifications.

What happened that warrants my question:

This month it was brought to my attention that one of my roommates has paid me a higher amount for rent over the last two months. The number she started using hasn’t been correct since sometime around last august. She requested i pay her the amount back she overpaid. She was emailed the proper rent amount when it changed like everyone else and she even paid it correctly the month before she randomly started paying more.

I totally understand now that I have to cross check everyone’s numbers from now on. Also, as a broke, full time student, I don’t question when I have a few extra bucks in the account after all bills are paid. My intuition tells me that she absolutely deserves to be compensated for this because I should have been checking numbers, but i also feel that since I properly informed her of the amount and she demonstrated competence in paying it properly already, I don’t owe her the full amount she overpaid. Nobody in the house understands how long it takes to calculate bills for 7 people every month (the audit of her payments alone has taken me an hour and a half today,) which I think counts for something.

I am definitely a bit out of my element here and want to handle this fairly. Any advice would be appreciated!