Ok, this situation just came up today, and I'm wondering how best to handle it because it's not at all clear cut. Two twin sisters are renting a 2 bedroom apartment from me, and the rent on that apartment went up by 3% from $660 to 680 effective May 1 (day after tomorrow). The first sister, Janet, paid her rent cheque yesterday in the amount of $330, which is $10 shy of the $340 she should have paid for her half of May's rent. So, I gave her a receipt for $340, but included a copy of the lease renewal form showing that the rent would be going up May 1 to $680. I figured I'd swallow the $10 just as long as she paid the correct rent next time and thereafter. Well today, her sister Janessa paid her share of the rent, and she ALSO paid $330. I can: 1. Pretend to be an idiot and give Janessa a receipt for $340 and ALSO remind her that the rent went up to $680, or (that's the "fair" option) 2. I can give Janessa her cheque back and tell her that I have already reminded them that the rent for may is $680, or (that's the "reasonable" option cuz I can't believe Janessa didn't see my note too) 3. I can give both cheques back and tell them both to pay $340 each. (that's the "right" option) What would you do and why?